Light-emitting element, light-emitting device, and electronic device

ABSTRACT

The light-emitting element includes: a light-emitting layer and a layer for controlling the movement of carriers between a first electrode and a second electrode. The layer for controlling the movement of carriers contains a first organic compound and a second organic compound, and is provided between the light-emitting layer and the second electrode. The first organic compound has an electron transporting property, and the second organic compound has an electron trapping property. The weight percent of the first organic compound is higher than that of the second organic compound. The light-emitting layer emits light when a voltage is applied such that the potential of the first electrode is higher than that of the second electrode. The first organic compound having the electron transporting property may be replaced with an organic compound having a hole transporting property, and the second organic compound having the electron trapping property may be replaced with an organic compound having a hole trapping property.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to current-excitation light-emittingelements, and also relates to light-emitting devices and electronicdevices having such light-emitting elements.

2. Description of the Related Art

In recent years, a light-emitting element utilizing electroluminescencehas actively been researched and developed. The basic structure of thelight-emitting element is such that a light-emissive compound issandwiched between a pair of electrodes. By applying a voltage to suchan element, light emission can be obtained from the light-emissivecompound.

Such a light-emitting element which is a self-luminous type has anadvantage in that it has higher visibility of pixels than liquid crystaldisplays, and there is no need to use a backlight. Thus, such alight-emitting element is considered to be suitable for a flat paneldisplay element. Besides, such a light-emitting element has advantagesin that it can be formed to be thin and lightweight, and has quite fastresponse speed.

Furthermore, since such a light-emitting element can be formed in a filmform, planar light emission can be easily obtained by forming alarge-area element. This cannot be easily achieved with a point lightsource typified by an incandescent lamp or an LED, or with a line lightsource typified by a fluorescent lamp. Therefore, the light-emittingelement has a high utility value as a plane light source that can beapplied to lighting or the like.

The light-emitting elements using electroluminescence can be roughlyclassified into light-emitting elements whose light-emissive compound isan organic compound and light-emitting elements whose light-emissivecompound is an inorganic compound. The invention relates to the formerlight-emitting elements whose light-emissive compound is an organiccompound. In this case, when a voltage is applied to the light-emittingelement, electrons and holes are injected from a pair of electrodes intoa layer containing a light-emissive organic compound, whereby a currentflows thereto. Then, the carriers (electrons and holes) are recombinedand the light-emissive organic compound comes to an excited state. Whenthe organic compound returns from the excited state to the ground state,light emission is obtained.

Because of such a mechanism, the light-emitting element is called acurrent-excitation light-emitting element. As the types of the excitedstates obtained by an organic compound, there are a singlet excitedstate and a triplet excited state. Light emission from the singletexcited state is referred to as fluorescence, and light emission fromthe triplet excited state is referred to as phosphorescence.

Such a light emitting element has many material-dependent problems forimprovement of the element characteristics, and in order to overcome theproblems, improvement in element structure, development of materials,and the like have been conducted. For example, in Reference 1 (TetsuoTSUTSUI, and eight others, Japanese Journal of Applied Physics, Vol. 38,L1502-L1504 (1999)), the luminous efficiency of a light-emitting elementusing a phosphorescent material is improved by providing a hole blockinglayer.

However, since the hole blocking layer disclosed in Reference 1 is notdurable, the light-emitting element has a short lifetime. Therefore,improvement in lifetime of the light-emitting element is desired. Inview of the foregoing, it is an object of the invention to provide alight-emitting element having a long lifetime. It is another object ofthe invention to provide a light-emitting device and an electronicdevice having a long lifetime.

SUMMARY OF THE INVENTION

As a result of diligent study, the inventors found that changes incarrier balance over time can be suppressed by providing a layer forcontrolling the movement of carriers, i.e., a layer for controlling themovement of electrons or holes. The inventors also found that along-lifetime light-emitting element can be obtained by the provision ofsuch a layer.

Thus, one aspect of a light-emitting element of the invention includes:a first electrode, a second electrode, a light-emitting layer, and alayer for controlling the movement of carriers. The light-emitting layerand the layer for controlling the movement of carriers are sandwichedbetween the first electrode and the second electrode, the layer forcontrolling the movement of carriers contains a first organic compoundand a second organic compound, the layer for controlling the movement ofcarriers is provided between the light-emitting layer and the secondelectrode, the first organic compound is an organic compound having anelectron transporting property, the second organic compound is anorganic compound having an electron trapping property, the weightpercent of the first organic compound is higher than the weight percentof the second organic compound in the layer for controlling the movementof carriers, and the light-emitting layer emits light when a voltage isapplied such that the potential of the first electrode is higher thanthe potential of the second electrode.

In the above structure, the lowest unoccupied molecular orbital level ofthe second organic compound is preferably lower than the lowestunoccupied molecular orbital level of the first organic compound by 0.3eV or more. Further, the light-emitting layer preferably has an electrontransporting property. For example, it is preferable that thelight-emitting layer contain a third organic compound and a fourthorganic compound, the weight percent of the third organic compound behigher than the weight percent of the fourth organic compound, and thethird organic compound have an electron transporting property. Inaddition, the first organic compound and the third organic compound arepreferably different organic compounds. In the above structure, thefirst organic compound is preferably a metal complex. In addition, thesecond organic compound is preferably contained in the layer forcontrolling the movement of carriers in the range of 0.1 wt % to 5 wt %or in the range of 0.1 mol % to 5 mol %. In addition, in the abovestructure, the second organic compound is preferably coumarinderivatives.

One aspect of a light-emitting element of the invention includes: afirst electrode, a second electrode, a light-emitting layer, and a layerfor controlling the movement of carriers. The light-emitting layer andthe layer for controlling the movement of carriers are sandwichedbetween the first electrode and the second electrode, the layer forcontrolling the movement of carriers contains a first organic compoundand a second organic compound, the layer for controlling the movement ofcarriers is provided between the light-emitting layer and the firstelectrode, the first organic compound is an organic compound having ahole transporting property, the second organic compound is an organiccompound having a hole trapping property, the weight percent of thefirst organic compound is higher than the weight percent of the secondorganic compound in the layer for controlling the movement of carriers,and the light-emitting layer emits light when a voltage is applied suchthat the potential of the first electrode is higher than the potentialof the second electrode.

In the above structure, the highest unoccupied molecular orbital levelof the second organic compound is preferably higher than the highestunoccupied molecular orbital level of the first organic compound by 0.3eV or more. Further, the light-emitting layer preferably has a holetransporting property. For example, it is preferable that thelight-emitting layer contain a third organic compound and a fourthorganic compound, the weight percent of the third organic compound behigher than the weight percent of the fourth organic compound, and thethird organic compound have a hole transporting property. In addition,the first organic compound and the third organic compound are preferablydifferent organic compounds. The second compound is preferably containedin the layer for controlling the movement of carriers in the range of0.1 wt % to 5 wt % or in the range of 0.1 mol % to 5 mol %. Accordingly,the amount of trapped carriers can be controlled appropriately. Further,in the above structure, the first organic compound is preferably anaromatic amine compound.

In the above structure, the thickness of the layer for controlling themovement of carriers is preferably in the range of 5 to 20 nm. That is,the thickness of the layer for controlling the movement of carriers ispreferably in the range of 5 to 20 nm regardless of whether the firstorganic compound having a carrier transporting property has an electrontransporting property or a hole transporting property or regardless ofwhether the compound having a carrier trapping property has an electrontrapping property or a hole trapping property. In any case, the layerfor controlling the movement of carriers is preferably provided to be incontact with the light-emitting layer.

In addition, the invention includes a light-emitting device whichincludes the above-described light-emitting element. The light-emittingdevice in this specification includes all of image display devices,light-emitting devices, and light sources (including lighting devices).In addition, the invention also includes a module in which a connectorsuch as an FPC (Flexible Printed Circuit), TAB (Tape Automated Bonding)tape, or a TCP (Tape Carrier Package) is attached to a panel on whichlight-emitting elements are formed, a module in which a printed wiringboard is connected to a tip of the TAB tape or TCP, and a module inwhich an IC (Integrated Circuit) is directly mounted on thelight-emitting elements by a COG (Chip On Glass) method.

Further, the invention also includes an electronic device having adisplay portion which includes the light-emitting element of theinvention. Therefore, an electronic device of the invention includes adisplay portion which includes the above-described light-emittingelement and a controller for controlling the emission of thelight-emitting element.

As described above, in the invention, a layer for controlling themovement of carriers is formed by combining a first organic compoundhaving a carrier transporting property (i.e., an electron transportingproperty or a hole transporting property) and a second organic compoundhaving a carrier trapping property (i.e., an electron trapping propertyor a hole trapping property). The carrier trapping property as referredto in this specification is not an absolute property, but is a relativeproperty with respect to the carrier transporting property. That is, thecarrier trapping property has a function of reducing the carriertransporting property of the first organic compound which is used in thelayer for controlling the movement of carriers within a predeterminedrange.

For determination of the properties, when the first organic compound hasan electron trapping property, the lowest unoccupied molecular orbitallevel may be used for reference, whereas when the first organic compoundhas a hole transporting property, the highest unoccupied molecularorbital level may be used for reference. For example, when the firstorganic compound has an electron transporting property, it is preferablethat the lowest unoccupied molecular orbital level of the second organiccompound which has an electron trapping property be lower than that ofthe first organic compound by 0.3 eV or more. Meanwhile, when the firstorganic compound has a hole transporting property, it is preferable thatthe highest unoccupied molecular orbital level of the second organiccompound which has a hole trapping property be higher than that of thefirst organic compound by 0.3 eV or more.

The light-emitting element of the invention has a layer for controllingthe movement of carriers, whereby changes in carrier balance over timecan be suppressed. Therefore, a long-lifetime light-emitting element canbe obtained. Further, when the light-emitting element of the inventionis applied to a light-emitting device and an electronic device, it ispossible to provide a light-emitting device and an electronic devicehaving high luminous efficiency and reduced power consumption. Inaddition, a light-emitting device and electronic device having a longlifetime can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B illustrate light-emitting elements of the inventionshown in Embodiment Mode 1;

FIGS. 2A and 2B illustrate light-emitting elements of the inventionhaving different structures from those in FIGS. 1A and 1B of EmbodimentMode 1;

FIGS. 3A to 3C illustrate examples of the light emission of thelight-emitting element of the invention shown in Embodiment Mode 1;

FIG. 4 illustrates an example of a band diagram of the light-emittingelement of the invention shown in FIG. 1A;

FIGS. 5A and 5B illustrate light-emitting elements of the inventionshown in Embodiment Mode 2;

FIGS. 6A and 6B illustrate light-emitting elements of the inventionhaving different structures from those in FIGS. 5A and 5B of EmbodimentMode 2;

FIGS. 7A to 7C illustrate examples of the light emission of thelight-emitting element of the invention shown in Embodiment Mode 2;

FIG. 8 illustrates an example of a band diagram of the light-emittingelement of the invention shown in FIG. 5A;

FIG. 9 illustrates a light-emitting element in which a plurality oflight-emitting units in Embodiment Mode 3 is stacked;

FIGS. 10A and 10B illustrate an active matrix light-emitting device ofthe invention shown in Embodiment Mode 4;

FIGS. 11A and 11B illustrate a passive matrix light-emitting device ofthe invention shown in Embodiment Mode 4;

FIGS. 12A to 12D illustrate electronic devices of the invention;

FIG. 13 illustrates an electronic device which uses the light-emittingdevice of the invention as a backlight;

FIG. 14 illustrates a table lamp which uses the lighting device of theinvention;

FIG. 15 illustrates an indoor lighting device which uses the lightingdevice of the invention;

FIG. 16 illustrates a light-emitting element of an embodiment.

FIG. 17 shows the current density vs. luminance characteristics of alight-emitting element 1;

FIG. 18 shows the voltage vs. luminance characteristics of thelight-emitting element 1;

FIG. 19 shows the luminance vs. current efficiency characteristics ofthe light-emitting element 1;

FIG. 20 shows the emission spectrum of the light-emitting element 1;

FIG. 21 shows the results of continuous lighting tests in which alight-emitting element 1 and a reference light-emitting element 2 werecontinuously lit by constant current driving;

FIG. 22 shows the current density vs. luminance characteristics of alight-emitting element 3;

FIG. 23 shows the voltage vs. luminance characteristics of thelight-emitting element 3;

FIG. 24 shows the luminance vs. current efficiency characteristics ofthe light-emitting element 3;

FIG. 25 shows the emission spectrum of the light-emitting element 3;

FIG. 26 shows the results of continuous lighting tests in which thelight-emitting element 3, a light-emitting element 4, a light-emittingelement 5, and a reference light-emitting element 6 were continuouslylit by constant current driving;

FIG. 27 shows the current density vs. luminance characteristics of thelight-emitting element 4;

FIG. 28 shows the voltage vs. luminance characteristics of thelight-emitting element 4;

FIG. 29 shows the luminance vs. current efficiency characteristics ofthe light-emitting element 4;

FIG. 30 shows the emission spectrum of the light-emitting element 4;

FIG. 31 shows the current density vs. luminance characteristics of thelight-emitting element 5;

FIG. 32 shows the voltage vs. luminance characteristics of thelight-emitting element 5;

FIG. 33 shows the luminance vs. current efficiency characteristics ofthe light-emitting element 5;

FIG. 34 shows the emission spectrum of the light-emitting element 5;

FIG. 35 shows the reduction reaction characteristics of Alq;

FIG. 36 shows the reduction reaction characteristics of DPQd;

FIG. 37 shows the reduction reaction characteristics of Coumarin 6;

FIG. 38 shows the current density vs. luminance characteristics of alight-emitting element 7;

FIG. 39 shows the voltage vs. luminance characteristics of thelight-emitting element 7;

FIG. 40 shows the luminance vs. current efficiency characteristics ofthe light-emitting element 7;

FIG. 41 shows the emission spectrum of the light-emitting element 7;

FIG. 42 shows the result of a continuous lighting test in which thelight-emitting element 7 was continuously lit by constant currentdriving;

FIG. 43 shows the current density vs. luminance characteristics of alight-emitting element 8;

FIG. 44 shows the voltage vs. luminance characteristics of thelight-emitting element 8;

FIG. 45 shows the luminance vs. current efficiency characteristics ofthe light-emitting element 8;

FIG. 46 shows the emission spectrum of the light-emitting element 8;

FIG. 47 shows the result of a continuous lighting test in which thelight-emitting element 8 was continuously lit by constant currentdriving;

FIG. 48 illustrates a light-emitting element of an embodiment;

FIG. 49 shows the current density vs. luminance characteristics of alight-emitting element 9;

FIG. 50 shows the voltage vs. luminance characteristics of thelight-emitting element 9;

FIG. 51 shows the luminance vs. current efficiency characteristics ofthe light-emitting element 9;

FIG. 52 shows the emission spectrum of the light-emitting element 9;

FIG. 53 shows the result of a continuous lighting test in which thelight-emitting element 9 was continuously lit by constant currentdriving;

FIG. 54 shows the current density vs. luminance characteristics of alight-emitting element 10;

FIG. 55 shows the voltage vs. luminance characteristics of thelight-emitting element 10;

FIG. 56 shows the luminance vs. current efficiency characteristics ofthe light-emitting element 10;

FIG. 57 shows the emission spectrum of the light-emitting element 10;

FIG. 58 shows the result of a continuous lighting test in which thelight-emitting element 10 was continuously lit by constant currentdriving;

FIG. 59 shows the current density vs. luminance characteristics of alight-emitting element 11;

FIG. 60 shows the voltage vs. luminance characteristics of thelight-emitting element 11;

FIG. 61 shows the luminance vs. current efficiency characteristics ofthe light-emitting element 11;

FIG. 62 shows the emission spectrum of the light-emitting element 11;

FIG. 63 shows the result of a continuous lighting test in which thelight-emitting element 11 was continuously lit by constant currentdriving;

FIG. 64 shows the reduction reaction characteristics of BPAPQ; and

FIG. 65 shows the reduction reaction characteristics of DNTPD.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment modes of the invention which include the best mode forcarrying out the invention will be described in detail below withreference to the accompanying drawings. Note that the invention is notlimited to the following description, and it will be easily understoodby those skilled in the art that various changes and modifications canbe made without departing from the spirit and scope of the invention.Therefore, the invention should not be construed as being limited to thedescription in the following embodiment modes.

First, luminance decay factors of light-emitting elements will bedescribed. Light-emitting elements are generally and often driven with aconstant current. In that case, luminance decay means a decrease incurrent efficiency. The current efficiency means the proportion ofoutput light relative to a current flow. Therefore, the currentefficiency is greatly influenced by how many of carriers that areflowing in the light-emitting element can be recombined in alight-emitting layer (carrier balance) or how many of carriers that havebeen recombined in the light-emitting layer (i.e., exciton) cancontribute to light-emission (quantum yield).

Therefore, it is considered that changes in carrier balance over time ora decrease in quantum yield over time is the major factor of theluminance decay. In view of the foregoing, the invention has been madeby focusing on the changes in carrier balance over time. Thus,development of a light-emitting element in which changes in carrierbalance over time can be suppressed could be obtained.

Embodiment Mode 1

One example of a light-emitting element of the invention will bedescribed with reference to FIG. 1A. This embodiment mode illustrates alight-emitting element which includes a layer for controlling themovement of electrons as a layer for controlling the movement ofcarriers. That is, in the invention, changes in carrier balance overtime are suppressed by using the layer for controlling the movement ofcarriers, so that the carriers are recombined at a position away fromelectrodes, whereby the lifetime of the light-emitting element isprolonged.

The light-emitting element of the invention has a plurality of layersbetween a pair of electrodes. The plurality of layers is stacked bycombining layers made of a compound with a high carrier injectionproperty and a compound with a high carrier transporting property sothat a light-emitting region is formed at a position away from theelectrodes, i.e., so that carriers are recombined at a position awayfrom the electrodes.

In this embodiment mode, a light-emitting element includes a firstelectrode 202, a second electrode 204, and an EL layer 203 providedbetween the first electrode 202 and the second electrode 204. Note thatin this embodiment mode, description will be made on the assumption thatthe first electrode 202 functions as an anode and the second electrode204 functions as a cathode. That is, light emission is obtained when avoltage is applied to the first electrode 202 and the second electrode204 so that the potential of the first electrode 202 is higher than thepotential of the second electrode 204.

The substrate 201 is used as a support of the light-emitting element.The substrate 201 can be made of glass, plastic, or the like, forexample. Note that the substrate 201 of the light-emitting element ofthe invention may remain in a light-emitting device or an electronicdevice which is a product utilizing the light-emitting element.Alternatively, the substrate 201 may be used merely as a support in theprocess of forming the light-emitting element. In that case, thesubstrate 201 does not remain in a finished product.

The first electrode 202 is preferably formed using a material with ahigh work function (i.e., 4.0 eV or higher) such as metals, alloys,electrically conductive compounds, or a mixture of them. Specifically,indium tin oxide (ITO), ITO containing silicon or silicon oxide, indiumzinc oxide (IZO), indium oxide containing tungsten oxide and zinc oxide(IWZO), and the like can be given.

Such conductive metal oxide films are generally formed by sputtering,but may also be formed by an ink-jet method, a spin coating method, orthe like by application of a sol-gel method or the like. For example,indium zinc oxide (IZO) can be deposited by a sputtering method using atarget in which 1 to 20 wt % of zinc oxide is added to indium oxide. Inaddition, indium oxide containing tungsten oxide and zinc oxide (IWZO)can be deposited by a sputtering method using a target in which 0.5 to 5wt % of tungsten oxide and 0.1 to 1 wt % of zinc oxide are added toindium oxide. Further, gold (Au), platinum (Pt), nickel (Ni), tungsten(W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper(Cu), palladium (Pd), titanium (Ti), nitride of metal materials (e.g.,titanium nitride), and the like can be used.

When a layer containing a composite material which will be describedlayer is used as a layer having a contact with the first electrode, thefirst electrode can be formed using various metals, alloys, electricallyconductive compound, a mixture of them, or the like regardless of theirwork functions. For example, aluminum (Al), silver (Ag), an aluminumalloy (AlSi), or the like can be used. Note that in this specification,the term “composite” means the state that charges can be transferredbetween materials by not only simple mixture of two materials but alsoby mixture of a plurality of materials.

Besides, an element belonging to Group 1 or 2 of the periodic tablewhich has a low work function, i.e., alkali metals such a lithium (Li)and cesium (Cs) and alkaline earth metals such as magnesium (Mg),calcium (Ca), and strontium (Sr); alloys of them (e.g., MgAg and AlLi);rare earth metals such as europium (Eu) and ytterbium (Yb); alloys ofthem; and the like can also be used. A film made of an alkali metal, analkaline earth metal, or an alloy of them can be formed by a vacuumdeposition method. Further, a film made of an alloy of an alkali metalor an alkaline earth metal can be formed by a sputtering method. It isalso possible to deposit a silver paste or the like by an ink-jet methodor the like.

The EL layer 203 includes a first layer 211, a second layer 212, a thirdlayer 213, a fourth layer 214, a fifth layer 215, and a sixth layer 216.In the EL layer 203, the third layer 213 is a light-emitting layer andthe fourth layer 214 is a layer for controlling the movement ofcarriers. Note that it is acceptable as long as the EL layer 203includes a layer for controlling the movement of carriers and alight-emitting layer shown in this embodiment mode. Thus, the structureof the other stacked layers is not specifically limited. For example,the EL layer 203 can be formed by appropriate combination of a holeinjection layer, a hole transporting layer, a light-emitting layer, alayer for controlling the movement of carriers, an electron transportinglayer, an electron injection layer, and the like.

The first layer 211 is a layer containing a compound with a high holeinjection property. As a compound with a high hole injection property,molybdenum oxide, vanadium oxide, ruthenium oxide, tungsten oxide,manganese oxide, and the like can be used. Besides, phthalocyaninecompounds such as phthalocyanine (abbreviation: H₂PC), copper(II)phthalocyanine (abbreviation: CuPc), and vanadyl(IV) phthalocyanine(VOPc) can also be given as low molecular organic compounds.

Further, the following low molecular organic compounds can be used:aromatic amine compounds such as4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1).

Alternatively, the first layer 211 can be formed using a compositematerial in which a compound with an acceptor property is mixed into acompound with a high hole transporting property. Note that when acomposite material in which a compound with an acceptor property ismixed into a compound with a high hole transporting property is used, amaterial for forming the electrode can be selected regardless of itswork function. That is, not only a material with a high work function,but also a material with a low work function can be used for the firstelectrode 202. Such a composite material can be formed by co-depositinga compound with a high hole transporting property and a compound with anacceptor property.

As an organic compound used for the composite material, variouscompounds such as aromatic amine compounds, carbazole derivatives,aromatic hydrocarbons, and high molecular compounds (e.g., oligomer,dendrimer, or polymer) can be used. The organic compound used for thecomposite material is preferably an organic compound having a high holetransporting property. Specifically, a compound having a hole mobilityof 10⁻⁶ cm²/Vs or higher is preferably used. However, other compoundsmay also be used as long as the hole transporting properties thereof arehigher than the electron transporting properties thereof. Specificorganic compounds that can be used for the composite material aredescribed below.

For example, the following organic compounds can be used for thecomposite material: aromatic amine compounds such as MTDATA, TDATA,DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD), andN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD); and carbazole derivatives such as4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), and1,4-bis[4-(N-carbazolyl)phenyl-2,3,5,6-tetraphenylbenzene.

Alternatively, the following aromatic hydrocarbon compounds can also beused: 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl)-2-tert-butyl-anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, and the like.

Further, the following aromatic hydrocarbon compounds can also be used:2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation:DPVPA), and the like.

As a compound with an acceptor property, organic compounds such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ) and chloranil, and transition metal oxide can be given. Inaddition, oxide of metals belonging to Groups 4 to 8 in the periodictable can also be given. Specifically, it is preferable to use vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide which have highelectron accepting properties. Above all, molybdenum oxide isparticularly preferable because it is stable even in atmospheric air,has a low hygroscopic property, and is easy to handle.

For the first layer 211, high molecular compounds (e.g., oligomer,dendrimer, or polymer) can be used. For example, the following highmolecular compounds can be used: poly(N-vinylcarbazole) (abbreviation:PVK), poly(4-vinyl triphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine (abbreviation:Poly-TPD). Further, high molecular compounds mixed with acid such aspoly (3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) andpolyaniline/poly(styrenesulfonate) (PAni/PSS) can also be used. Notethat it is also possible to form the first layer 211 using a compositematerial which is formed from the above-described high molecularcompound such as PVK, PVTAP, PTPDMA, or Poly-TPD and the above-describedcompound having an acceptor property.

The second layer 212 is a layer containing a compound with a high holetransporting property. As a compound with a high hole transportingproperty, the following low molecular organic compounds can be used:aromatic amine compounds such as NPB (or α-NPD), TPD,4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB). The compounds described here are mainly compoundshaving a hole mobility of 10⁻⁶ cm²/Vs or higher.

Further, other compounds may also be used for the second layer 212 aslong as the hole transporting properties thereof are higher than theelectron transporting properties thereof. Note that the layer containinga compound with a high hole transporting property is not limited to asingle layer but may have a stacked structure of two or more layers madeof the above-described compounds. Further, the second layer 212 may alsobe formed with high molecular compounds such as PVK, PVTPA, PTPDMA, andPoly-TPD.

The third layer 213 is a layer containing a highly light-emissivecompound, which corresponds to the light-emitting layer of theinvention. The third layer 213 can be formed using various materialssuch as low molecular organic compounds. Specifically, as alight-emitting material which exhibits bluish light, the following canbe used:N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: (YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA), and the like.

As a light-emitting material which exhibits greenish light emission, thefollowing can be used:N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamineabbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), and the like.

As a light-emitting material which exhibits yellowish light emission,rubrene, 5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene(abbreviation: BPT), and the like can be used. Further, as alight-emitting material which exhibits reddish light emission,N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD);7,13-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-α]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD), and the like can be used. Alternatively, aphosphorescent material such asbis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate (abbreviation: Ir(btp)₂(acac)) can also be used.

Note that in this embodiment mode, the layer for controlling themovement of carriers is provided between the light-emitting layer andthe second electrode functioning as the cathode. Therefore, thelight-emitting layer preferably has an electron transporting property.Conventionally, when a light-emitting layer has an electron transportingproperty, an electron blocking layer has been provided between thelight-emitting layer and an anode in order to prevent electrons frompenetrating the light-emitting layer. However, when the electronblocking layer has deteriorated over time, a recombination regionexpands to the inside of the electron blocking layer (or inside of thehold transporting layer), which could result in a significant decreasein current efficiency (i.e., luminance decay). Meanwhile, in theinvention, the movement of electrons is controlled before the electronsreach the light-emitting layer (between the light-emitting layer and thecathode). Therefore, even when the balance of electrons (e.g., mobilityor the amount of electrons relative to that of holes) is somewhat lost,the proportion of recombination in the light-emitting layer hardlychanges, which is advantageous in that luminance does not easily decay.

Note that the light-emitting layer may also have a structure in whichthe above-described highly light-emissive compound is dispersed inanother compound. Various compounds can be used for the material inwhich the light-emissive compound is dispersed. In particular, it ispreferable to use a compound whose lowest unoccupied molecular orbital(LUMO) level is higher than that of the light-emissive compound andwhose highest occupied molecular orbital (HOMO) level is lower than thatof the light-emissive compound.

Note that in this embodiment mode, the light-emitting layer preferablyhas an electron transporting property because the layer for controllingthe movement of carriers is provided between the light-emitting layerand the second electrode functioning as the cathode. That is, theelectron transporting property of the light-emitting layer is preferablyhigher than the hole transporting property thereof. Therefore, theabove-described material in which the highly light-emissive compound isdispersed is preferably an organic compound having an electrontransporting property.

Specifically, the following metal complexes can be used:tris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (BAlq),bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (Abbreviation: ZnPBO), andbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂).

Further, the following heterocyclic compounds can also be used:2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(4-tert-buthylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), andbathocuproin (abbreviation: BCP).

Alternatively, the following condensed aromatic compounds can also beused: 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:CzPA), 3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilben-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilben-4,4′-diyl)diphenanthrene (abbreviation: DPNS2), and3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: DPB3).

As a material in which the light-emissive compound is dispersed, aplurality of kinds of materials can be used. For example, a compound forcontrolling the crystallization of rubrene or the like can be furtheradded in order to control the crystallization. In addition, NPB, Alq, orthe like can be further added in order to efficiently transfer energy tothe light-emissive compound. When a structure in which a highlylight-emissive compound is dispersed in another compound is employed,the crystallization of the third layer 213 can be suppressed. Further,concentration quenching which results from the high concentration of thelight-emissive compound can also be suppressed.

Further, high molecular compounds can be used for the third layer 213that is the light-emitting layer. Specifically, as a light-emittingmaterial which exhibits bluish light emission, the following can beused: poly(9,9-dioctylfluorene-2,7-diyl) (abbreviation: POF),poly[(9,9-dioctylfluorene-2,7-diyl-co-(2,5-dimethoxybenzene-1,4-diyl)](abbreviation: RF-DMOP),poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-di-(p-butylphenyl)-1,4-diaminobenzene]}(abbreviation: TAB-PFH), and the like.

As a light-emitting material which exhibits greenish light emission, thefollowing can be used: poly(p-phenylenevinylene) (abbreviation: PPV),poly[(9,9-dihexylfluorene-2,7-diyl)-alt-co-(benzo[2,1,3]thiadiazol-4,7-diyl)](abbreviation: PFBT),poly[(9,9-dioctylfluorene-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)],and the like.

As a light-emitting material which exhibits orangish to reddish lightemission, the following can be used:poly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene] (abbreviation:MEH-PPV), poly(3-butylthiophene-2,5-diyl),poly{[9,9-dihexyl-2,7-bis(1-cyanovinylene)fluorenylene]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]},poly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-bis(1-cyanovinylenephenylene)]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]}(abbreviation: CN-PPV-DPD), and the like.

The fourth layer 214 is a layer for controlling the movement ofcarriers, and contains at least two kinds of compounds. The mostimportant feature of the invention is that the layer for controlling themovement of carriers is provided, and also the layer includes a firstorganic compound which is an organic compound having a carriertransporting property and a second organic compound having a carriertrapping property. The organic compound having a carrier transportingproperty is categorized as a compound having an electron transportingproperty or a compound having a hole transporting property. Similarly,the organic compound having a carrier transporting property iscategorized as a compound having an electron trapping property or acompound having a hole trapping property.

The fourth layer 214 in this embodiment mode contains a higher weightpercent of the first organic compound than the weight percent of thesecond organic compound. This embodiment mode will describe the casewhere the layer for controlling the movement of carriers is provided ata position closer to the second electrode functioning as the cathodethan the layer having a light-emitting function (i.e., thelight-emitting layer) is. That is, a case where the layer forcontrolling the movement of carriers is provided between the third layer213 having the light-emitting function and the second electrode 204 willbe described.

When the layer for controlling the movement of carriers is provided at aposition closer to the second electrode functioning as the cathode thanthe light-emitting layer is, the first organic compound is preferably anorganic compound having an electron transporting property. That is, thefirst organic compound is preferably a compound whose electrontransporting property is higher than the hole transporting property. Inaddition, the second organic compound is preferably an organic compoundhaving a function of trapping electrons. That is, the second organiccompound is preferably an organic compound whose lowest unoccupiedmolecular orbital (LUMO) level is lower than that of the first organiccompound by 0.3 eV or more. When the layer for controlling the movementof carriers includes the second organic compound, the electrontransporting speed of the layer as a whole can be lower as compared withthe case where the layer is made of only the first organic compound.That is, adding the second organic compound makes it possible to controlthe movement of carriers. Further, controlling the concentration of thesecond organic compound makes it possible to control the movement speedof carriers. The concentration of the second organic compound ispreferably in the range of 0.1 wt % to 5 wt % or in the rage of 0.1 mol% to 5 mol %.

It is preferable that the emission colors of the light-emitting layerand the second organic compound be similar colors. For example, when theorganic compound contained in the light-emitting layer is an organiccompound which exhibits bluish light emission such as YGA2S or YGAPA,the second organic compound is preferably a compound which exhibitslight emission in the range of blue to bluish green such as acridone,Coumarin 102, Coumarin 6H, Coumarin 480D, or Coumarin 30. In thismanner, even if the second organic compound unintendedly emits light,the color purity of the emitted light can be in a good condition.

In addition, when the organic compound contained in the light-emittinglayer is an organic compound which exhibits greenish light emission suchas 2PCAPA, 2PCABPhA, 2DPAPA, 2DPABPhA, 2YGABPhA, or DPhAPhA, the secondorganic compound is preferably a compound which exhibits light emissionin the range of bluish green to yellowish green such asN,N′-dimethylquinacridone (abbreviation: DMQd),N,N′-diphenylquinacridone (abbreviation: DPQd),9,18-dihydro-benzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-1),9,18-dihydro-9,18-dimethyl-benzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-2), Coumarin 30, Coumarin 6, Coumarin 545T, orCoumarin 153.

Further, when the organic compound contained in the light-emitting layeris an organic compound which exhibits yellowish light emission such asrubrene or BPT, the second organic compound is preferably a compoundwhich exhibits light emission in the range of yellowish green to goldenyellow such as DMQd or(2-{2-[4-(9H-carbazol-9-yl)phenyl]ethenyl}-6-[2-(4-dimethylaminophenyl)ethenyl]-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCMCz).

Further, when the organic compound contained in the light-emitting layeris an organic compound which exhibits reddish light emission such asp-mPhTD or p-mPhAFD, the second organic compound is preferably acompound which exhibits light emission in the range of orange to redsuch as2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),{2-(1,1-dimethylethyl)-6-[2-(2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTB), orNile red.

Further, when the light-emitting material contained in thelight-emitting layer is a phosphorescent material, the second organiccompound is also preferably a phosphorescent material. For example, whenthe light-emitting material is the above-described Ir(btp)₂(acac) whichexhibits red light emission, the second organic compound may be a redphosphorescent material such as(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)). Note that the above-described compoundsare compounds having particularly low LUMO levels among compounds thatare used for light-emitting elements. Thus, when such compounds areadded to the first organic compound which will be described later, anexcellent trapping property can be obtained.

Note that among the above compounds exemplarily illustrated for thesecond organic compound, quinacridone derivatives such as DMQd, DPQd,DMNQd-1, and DMNQd-2 are chemically stable and thus preferable. That is,when quinacridone derivatives are used, the lifetime of thelight-emitting element can be particularly prolonged. In addition, sincequinacridone derivatives exhibit greenish light emission, the elementstructure of the light-emitting element of the invention is particularlyeffective for a light-emitting element of a greenish color. A greencolor requires the highest level of luminance in forming a full-colordisplay, and there are cases where the deterioration speed of a greenlight-emitting element is faster than those of other light-emittingelements. However, such a problem can be ameliorated by applying theinvention.

In addition, the first organic compound contained in the fourth layer214 is an organic compound having an electron transporting property.That is, the first organic compound is a compound whose electrontransporting property is higher than the hole transporting property.Specifically, the following can be used: metal complexes such as Alq,Almq₃, BeBq₂, BAlq, Znq, BAlq, ZnPBO, and ZnBTZ; heterocyclic compoundssuch as PBD, OXD-7, TAZ, TPBI, BPhen, and BCP; and condensed aromaticcompounds such as CzPA, DPCzPA, DPPA, DNA, t-BuDNA, BANT, DPNS, DPNS2,and TPB3.

Further, the following high molecular compounds can also be used:poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridin-3,5-diyl)](abbreviation: PF-Py) andpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-pyridin-6,6′-diyl)](abbreviation: PF-BPy). Above all, metal complexes that are stableagainst electrons are preferably used.

In addition, as mentioned earlier, the LUMO level of the second organiccompound is preferably lower than that of the first organic compound by0.3 eV or more. Therefore, it is acceptable as long as the first organiccompound is appropriately selected so as to satisfy the above conditionaccording to the kind of compound used for the second organic compound.For example, when DPQ or Coumarin 6 is used for the second organiccompound as will be later described in embodiments, the above conditioncan be satisfied by using Alq for the first organic compound.

Note that it is preferable that the emission colors of the highlylight-emissive compound contained in the third layer 213 and the secondorganic compound contained in the fourth layer 214 be similar colors.Therefore, it is preferable that a difference in peak values between theemission spectrum of the highly light-emissive compound and the emissionspectrum of the second organic compound be within the range of 30 nm.When the difference in peak values is within the range of 30 nm, theemission colors of the highly light-emissive compound and the secondorganic compound can be similar colors. Therefore, even when the secondorganic compound emits light due to changes in voltage or the like,changes in emission color can be suppressed.

Note that the second organic compound does not necessarily have to emitlight. For example, when the highly light-emissive compound has higherluminous efficiency than the second organic compound, it is preferableto control the concentration of the second organic compound in thefourth layer 214 so that only the light emission of the highlylight-emissive compound can be substantially obtained (by setting theconcentration of the second organic compound to be slightly lower thanthat of the highly light-emissive compound so that the light emission ofthe second organic compound can be suppressed). In that case, theemission colors of the highly light-emissive compound and the secondorganic compound are similar colors (i.e., they have about the samelevel of energy gap). Therefore, there is little possibility that energywill transfer from the highly light-emissive compound toward the secondorganic compound, and thus high luminous efficiency can be obtained.

Note that the second organic compound is preferably coumarin derivativessuch as Coumarin 102, Coumarin 6H, Coumarin 480D, Coumarin 30, Coumarin6, Coumarin 545T, and Coumarin 153. Coumarin derivatives have lowelectron trapping properties. Therefore, the concentration of the secondorganic compound added into the first organic compound may be relativelyhigh. That is, the concentration of the second organic compound caneasily be controlled, and thus a layer for controlling the movement ofcarriers which has desired properties can easily be obtained. Further,since coumarin derivatives have high luminous efficiency, decrease inluminous efficiency of the entire light-emitting element can besuppressed even when the second organic compound emits light.

FIG. 4 exemplarily illustrates a band diagram of the light-emittingelement of the invention shown in FIG. 1A. In FIG. 4, holes injectedfrom the first electrode 202 pass through the first layer 211 containinga compound with a high hole injection property and further through thesecond layer 212 containing a compound with a high hole transportingproperty. Then, the holes are injected to the third layer 213 containinga highly light-emissive compound. On the other hand, electrons injectedform the second electrode 204 pass through the sixth layer 216containing a compound with a high electron injection property andfurther through the fifth layer 215 containing a compound with a highelectron transporting property. Then, the electrons are injected to thefourth layer 214 that is the layer for controlling the movement ofcarriers. The movement speed of the electrons injected to the layer forcontrolling the movement of carriers is retarded by the second organiccompound having an electron trapping property. The electrons whosemovement speed has been retarded are injected to the third layer 213containing a highly light-emissive compound, and then recombined withholes. Thus, light emission is obtained.

When the third layer 213 has an electron transporting property, forexample, the movement speed of the holes that are injected form thesecond layer 212 to the third layer 213 is retarded. In addition, themovement speed of the electrons that are injected from the fourth layer214 to the third layer 213 is even slow in the third layer 213 becauseit has already been retarded in the fourth layer 214. Therefore, holesof a slow movement speed and electrons of a slow movement speed arerecombined in the third layer 213, whereby the recombination probabilityis increased and luminous efficiency is improved.

In the case of a conventional light-emitting element which does notinclude the fourth layer 214, the movement speed of electrons is notretarded but the electrons are directly injected to the third layer 213.Thus, the electrons reach the vicinity of the interface between thesecond layer 212 and the third layer 213. Therefore, a light-emittingregion is formed in the vicinity of the interface between the secondlayer 212 and the third layer 213. In that case, there is a possibilitythat the electrons may reach and deteriorate the second layer 212.Further, when the amount of electrons that have reached the second layer212 is increased over time, the recombination probability in thelight-emitting layer is decreased over time, which leads to a shorterlifetime of the element (luminance decay over time).

In the light-emitting element of the invention, electrons injected fromthe second electrode 204 pass through the sixth layer 216 containing acompound with a high electron injection property and further through thefifth layer 215 containing a compound with a high electron transportingproperty. Then, the electrons are injected to the fourth layer 214 thatis the layer for controlling the movement of carriers. Here, the fourthlayer 214 has a structure in which the second organic compound having afunction of trapping electrons is added to the first organic compoundhaving an electron transporting property. Therefore, the movement speedof the electrons that are injected to the fourth layer 214 is retardedand the electron injection to the third layer 213 is controlled.

As a result, a light-emitting region, which has conventionally beenformed in the vicinity of the interface between the second layer 212containing a compound with a high hole transporting property and thethird layer 213, is formed around a region from the third layer 213 tothe vicinity of the interface between the third layer 213 and the fourthlayer 214. Therefore, there is low possibility that electrons may reachand deteriorate the second layer 212 which contains a compound with ahigh hole transporting property. Similarly, as for holes, there is alsolow possibility that holes may reach and deteriorate the fifth layer 215which contains a compound with a high electron transporting propertybecause the fourth layer 214 contains the first organic compound havingan electron transporting property.

Further, it is an important point of the invention that not merely acompound with low electron mobility is applied to the fourth layer 214but an organic compound having an a function of trapping electrons,i.e., an organic compound having a carrier trapping property is added toan organic compound having an electron transporting property, i.e., anorganic compound having a carrier transporting property. With such astructure, it becomes possible not only to control the electroninjection to the third layer 213 but also to suppress changes in thecontrolled amount of electron injection over time. Therefore, thelight-emitting element of the invention can prevent a phenomenon thatcarrier balance is lost over time, which could otherwise lower therecombination probability. Thus, the lifetime of the element can beimproved (luminance decay over time can be suppressed).

In the light-emitting element of the invention, the light-emittingregion is not formed at the interface between the light-emitting layerand the hole transporting layer or the interface between thelight-emitting layer and the electron transporting layer. Therefore,there is no adverse effect of deterioration which would otherwise becaused if the light-emitting region is positioned close to the holetransporting layer or the electron transporting layer. Further, changesin carrier balance over time (in particular, changes in amount ofelectron injection over time) can be suppressed. Therefore, along-lifetime light-emitting element which does not easily deterioratecan be obtained.

In addition, it is preferable that the emission colors of the secondorganic compound contained in the fourth layer 214 and the highlylight-emissive compound contained in the third layer 213 be similarcolors. Specifically, it is preferable that a difference in peak valuesbetween the emission spectrum of the second organic compound and thehighly light-emissive compound be within the range of 30 nm. When thedifference in peak values is within the range of 30 nm, the emissioncolors of the second organic compound and the highly light-emissivecompound can be similar colors. Therefore, even when the second organiccompound emits light due to changes in voltage or the like, changes inemission color can be suppressed. Note that the second organic compounddoes not necessarily have to emit light.

In addition, the thickness of the fourth layer 214 is preferably in therange of 5 to 20 nm. When the fourth layer 214 is too thick, themovement speed of the carriers becomes too slow, which could result inhigh driving voltage. When the fourth layer 214 is too thin, on theother hand, it is impossible to implement the function of controllingthe movement of carriers. Therefore, the thickness of the fourth layer214 is preferably in the range of 5 to 20 nm.

The fifth layer 215 is a layer containing a compound with a highelectron transporting property. For example, as a low molecular organiccompound, metal complexes such as Alq, Almq₃, BeBq₂, BAlq, ZnPBO, andZnBTZ can be used. Further, besides the metal complexes, heterocycliccompounds such as PBD, OXD-7, TAZ, TPBI, BPhen, and BCP can also beused. The compounds described here are mainly compounds having anelectron mobility of 10⁻⁶ cm²/Vs or higher. Further, compounds otherthan the above-described compounds may also be used for the electrontransporting layer as long as the electron transporting propertiesthereof are higher than the hole transporting properties thereof.Furthermore, the electron transporting layer is not limited to a singlelayer but may have a stacked structure of two or more layers made of theabove-described compounds.

Further, the fifth layer 215 can also be formed using a high molecularcompound. For example, the following can be used:poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridin-3,5-diyl)](abbreviation: PF-Py),poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-pyridin-6,6′-diyl)](abbreviation: PF-BPy), and the like.

The sixth layer 216 is a layer containing a compound with a highelectron injection property. As a compound with a high electroninjection property, alkali metals, alkaline earth metals, or compoundsof them can be used such as lithium fluoride (LiF), cesium fluoride(CsF), and calcium fluoride (CaF₂). For example, it is possible to use alayer made of a compound with an electron transporting property in whichan alkali earth metal, an alkaline earth metal, or a compound of them ismixed, such as a mixture of Alq and magnesium (Mg). Note that when thelayer made of a compound with an electron transporting property in whichan alkali earth metal, an alkaline earth metal, or a compound of them ismixed is used, electrons can be efficiently injected from the secondelectrode 204, which is preferable.

The second electrode 204 is preferably formed using a compound with alow work function (i.e., 3.8 eV or lower) such as metals, alloys,electrically conductive compounds, or a mixture of them. Specificexamples of such a cathode material include an element belonging toGroup 1 or 2 of the periodic table, i.e., alkali metals such a lithium(Li) and cesium (Cs) and alkaline earth metals such as magnesium (Mg),calcium (Ca), and strontium (Sr); alloys of them (e.g., MgAg and AlLi);rare earth metals such as europium (Eu) and ytterbium (Yb); alloys ofthem; and the like. A film made of an alkali metal, an alkaline earthmetal, or an alloy of them can be formed by a vacuum deposition method.Further, a film made of an alloy of an alkali metal or an alkaline earthmetal can be formed by a sputtering method. It is also possible todeposit a silver paste or the like by an ink-jet method or the like.

When the sixth layer 216 which is a layer having a function of promotingelectron injection is provided between the second electrode 204 and thefifth layer 215, the second electrode 204 can be formed using variousconductive materials such as Al, Ag, ITO, and ITO containing silicon orsilicon oxide, regardless of their work functions. Further, suchconductive materials can be deposited by a sputtering method, an ink-jetmethod, a spin coating method, or the like.

As a method forming the EL layer, various methods can be used regardlessof a dry process or a wet process. For example, a vacuum depositionmethod, an ink-jet method, a spin coating method, or the like can beused. Further, different deposition methods may be used for differentelectrodes or different layers. For example, among the above-describedmaterials, a high molecular compound may be selected to form the ELlayer by a wet process. Alternatively, a low molecular organic compoundmay be selected to form the EL layer by a wet process. Further, it isalso possible to form the EL layer by selecting a low molecular organiccompound and using a dry process such as a vacuum deposition method.Similarly, the electrodes can be formed by a wet process such as asol-gel process or by a wet process with a paste of a metal material.Alternatively, the electrodes can be formed by a dry process such as asputtering method or a vapor deposition method.

A specific method for forming the light-emitting element will bedescribed below. In the case where the light-emitting element of theinvention is applied to a display device and its light-emitting layer isselectively deposited according to each color, the light-emitting layeris preferably formed by a wet process. When the light-emitting layer isformed by an ink-jet method, selective deposition of the light-emittinglayer for each color can be easily performed even when a large substrateis used.

For example, the structure shown in FIG. 1A can be obtained by the stepsof: forming the first electrode by a sputtering method which is a dryprocess, forming the first layer by an ink-jet method or a spin coatingmethod which is a wet process, forming the second layer by a vacuumdeposition method which is a dry process, forming the third layer by anink-jet method which is a wet process, forming the fourth layer by aco-deposition method which is a dry process, forming the fifth layer andthe sixth layer by a vacuum deposition method which is a dry process,and forming the second electrode by an ink-jet method or a spin coatingmethod which is a wet process.

Alternatively, the structure shown in FIG. 1A may be obtained by thesteps of: forming the first electrode by a ink-jet method which is a wetprocess, forming the first layer by a vacuum deposition method which isa dry process, forming the second layer by an ink-jet method or a spincoating method which is a wet process, forming the third layer by anink-jet method which is a wet process, forming the fourth layer by anink-jet method or a spin coating method which is a wet process, formingthe fifth layer and the sixth layer by an ink-jet method or a spincoating method which is a wet process, and forming the second electrodeby an ink-jet method or a spin coating method which is a wet process.Note that the deposition methods are not limited to the above methods,and a wet process and a dry process may be combined as appropriate.

For example, the structure shown in FIG. 1A can be obtained by the stepsof: forming the first electrode by a sputtering method which is a dryprocess, forming the first layer and the second layer by an ink-jetmethod or a spin coating method which is a wet process, forming thethird layer which is a light-emitting layer by an ink-jet method whichis a wet process, forming the fourth layer by a co-deposition methodwhich is a dry process, forming the fifth layer and the sixth layer by avacuum deposition method which is a dry process, and forming the secondelectrode by a vacuum deposition method which is a dry process.

That is, it is possible to form the first layer to the third layer bywet processes on the substrate having the first electrode which isformed in advance in a desired shape, and form the fourth layer to thesecond electrode thereon by dry processes. By this method, the firstlayer to the third layer can be formed at atmospheric pressure and thethird layer can be selectively deposited according to each color withease. In addition, the fourth layer to the second electrode can beconsecutively formed in vacuum. Therefore, the process can be simplifiedand productivity can be improved.

The process will be exemplarily described below. First, PEDOT/PSS isdeposited as the first layer on the first electrode. Since PEDOT/PSS issoluble in water, it can be deposited as an aqueous solution by a spincoating method, an ink-jet method, or the like. The second layer is notprovided but the third layer is provided as the light-emitting layer onthe first layer. The light-emitting layer can be formed by an ink-jetmethod, using a solution in which a light-emissive compound is dissolvedin a solvent (e.g., toluene, dodecylbenzene, or a mixed solvent ofdodecylbenzene and tetralin) in which the first layer (PEDOT/PSS) thatis already formed will not be dissolved. Next, the fourth layer isformed on the third layer. When the fourth layer is formed by a wetprocess, the fourth layer should be formed using a solvent in which thefirst layer and the third layer that are already formed will not bedissolved. In that case, the selection range of solvents is limited.Therefore, using a dry process is easier to form the fourth layer. Thus,by consecutively forming the fourth layer to the second electrode invacuum by a vacuum deposition method which is a dry process, the processcan be simplified.

Meanwhile, a structure shown in FIG. 2A can be formed in the reverseorder of the above-described steps: forming the second electrode by asputtering method or a vacuum deposition method which is a dry process,forming the sixth layer and the fifth layer by a vacuum depositionmethod which is a dry process, forming the fourth layer by aco-deposition method which is a dry process, forming the third layer byan ink-jet method which is a wet process, forming the second layer andthe first layer by an ink-jet method or a spin coating method which is awet process, and forming the first electrode by an ink-jet method or aspin coating method which is a wet process. By this method, the secondelectrode to the fourth layer can be consecutively formed in vacuum bydry processes, and the third layer to the first electrode can be formedat atmospheric pressure. Therefore, the process can be simplified andproductivity can be improved.

In the light-emitting element of the invention having the abovestructure, a current flows due to a potential difference generatedbetween the first electrode 202 and the second electrode 204, wherebyholes and electrons are recombined in the EL layer 203 and lightemission is obtained. Light emission is extracted outside through one orboth of the first electrode 202 and the second electrode 204. Therefore,one or both of the first electrode 202 and the second electrode 204 is alight-transmissive electrode.

When only the first electrode 202 is a light-transmissive electrode,light emission is extracted from the substrate side through the firstelectrode 202 as shown in FIG. 3A. Meanwhile, when only the secondelectrode 204 is a light-transmissive electrode, light emission isextracted from a side opposite to the substrate side through the secondelectrode 204 as shown in FIG. 3B. When both of the first electrode 202and the second electrode 204 are light-transmissive electrodes, lightemission is extracted from both the substrate side and the side oppositeto the substrate side through the first electrode 202 and the secondelectrode 204 as shown in FIG. 3C.

Note that the structure of the layers provided between the firstelectrode 202 and the second electrode 204 is not limited to the abovestructure. Any structure other than the above structure can be employedas long as a light-emitting region for recombination of holes andelectrons is positioned away from the first electrode 202 and the secondelectrode 204, and also a layer for controlling the movement of carriersis provided so as to prevent quenching which would otherwise be causedby the proximity of the light-emitting region to metal.

That is, the stacked structure of the layers is not particularlylimited. It is acceptable as long as layers made of a compound with ahigh electron transporting property, a compound with a high holetransporting property, a compound with a high electron injectionproperty, a compound with a high hole injection property, and a compoundwith a bipolar property (a compound having both high electron and holetransporting properties) are appropriately combined with the layer forcontrolling the movement of carriers and the light-emitting layer thatare shown in this embodiment mode.

Note that the layer for controlling the movement of carriers shown inthis embodiment mode is a layer for controlling the movement ofelectrons. Therefore, it is preferably provided at a position closer tothe electrode functioning as the cathode than the light-emitting layeris. For example, as shown in FIG. 1B, a seventh layer 217 containing acompound with a high electron transporting property may be providedbetween the third layer 213 having the light-emitting function and thefourth layer 214 that is the layer for controlling the movement ofcarriers.

More preferably, the layer for controlling the movement of carriers isdesirably provided to be in contact with the light-emitting layer. Whenthe layer for controlling the movement of carriers is provided to be incontact with the light-emitting layer, electron injection to thelight-emitting layer can be directly controlled. Therefore, changes incarrier balance over time in the light-emitting layer can be controlledmore efficiently, whereby the lifetime of the element can be moreeffectively prolonged. In addition, since the seventh layer 217containing a compound with a high electron transporting property is notrequired, the process can be simplified.

Note that when the layer for controlling the movement of carriers isprovided to be in contact with the light-emitting layer, it ispreferable that the first organic compound contained in the layer forcontrolling the movement of carriers be different from an organiccompound which is contained in large quantities in the light-emittinglayer. In particular, when the light-emitting layer contains a compound(a third organic compound) in which a highly light-emissive compound isdispersed and a highly light-emissive compound (a fourth organiccompound), it is desirable that the third organic compound and the firstorganic compound be different organic compounds. With such a structure,the movement of carriers (in this embodiment mode, electrons) whichtravel from the layer for controlling the movement of carriers towardthe light-emitting layer can be controlled even between the firstorganic compound and the third organic compound. Thus, the advantageouseffect of providing the layer for controlling the movement of carrierscan be further increased.

In addition, the light-emitting element shown in FIG. 2A has a structurein which the second electrode 204 functioning as the cathode, the ELlayer 203, and the first electrode 202 functioning as the anode aresequentially stacked over the substrate 201. The EL layer 203 includesthe first layer 211, the second layer 212, the third layer 213, thefourth layer 214, the fifth layer 215, and the sixth layer 216. Thefourth layer 214 is provided at a position closer to the secondelectrode functioning as the cathode than the third layer 213 is.

In this embodiment mode, the light-emitting element is formed over asubstrate made of glass, plastic, or the like. When a plurality of suchlight-emitting elements is formed over one substrate, a passive matrixlight-emitting device can be formed. In addition, it is also possible toform, for example, thin film transistors (TFTs) over a substrate made ofglass, plastic, or the like and form light-emitting elements onelectrodes that are electrically connected to the TFTs. Accordingly, anactive matrix light-emitting device in which drive of the light-emittingelements is controlled with the TFTs can be formed.

Note that the structure of the TFTs is not particularly limited. Eitherstaggered TFTs or inversely staggered TFTs may be employed. In addition,a driver circuit formed on the TFT substrate may be constructed fromboth n-channel and p-channel TFTs or from one of n-channel and p-channelTFTs. Further, the crystallinity of a semiconductor film used forforming the TFTs is not specifically limited. Either an amorphoussemiconductor film or a crystalline semiconductor film may be used.

A light-emitting element of the invention includes a layer forcontrolling the movement of carriers. The layer for controlling themovement of carriers contains at least two kinds of compounds.Therefore, by controlling the compatibility of compounds, the mixtureratio thereof, the thickness of the layer, and the like, carrier balancecan be precisely controlled. Further, since the carrier balance can becontrolled by controlling the compatibility of compounds, the mixtureratio thereof, the thickness of the layer, and the like, carrier balancecan be more easily controlled than in a conventional light-emittingelement. That is, the movement of carriers can be controlled not bychanging the physical properties of the material but by controlling themixture ratio, the thickness of the layer, and the like.

When the carrier balance is improved by using the layer for controllingthe movement of carriers, the luminous efficiency of the light-emittingelement can be improved. Further, using the layer for controlling themovement of the carriers makes it possible to prevent excessiveelectrons from being injected and also prevent electrons frompenetrating the light-emitting layer to reach the hole transportinglayer or the hole injection layer. When electrons have reached the holetransporting layer or the hole injection layer, the recombinationprobability in the light-emitting layer decreases (i.e., carrier balanceis lost), which could result in the decrease in luminous efficiency overtime. That is, the lifetime of the light-emitting element becomesshorter.

However, by using the layer for controlling the movement of carriers asshown in this embodiment mode, it becomes possible to prevent excessiveelectrons from being injected and also prevent electrons frompenetrating the light-emitting layer to reach the hole transportinglayer or the hole injection layer. Therefore, a decrease in luminousefficiency over time can be suppressed. That is, a long-lifetimelight-emitting element can be obtained. To be more specific, between thetwo or more kinds of compounds contained in the layer for controllingthe movement of carriers, the second organic compound which has a lowerweight percent than the weight percent of the first organic compound isused for controlling the movement of carriers. Therefore, the movementof carriers can be controlled with a component having the lowest weightpercent of all of the components contained in the layer for controllingthe movement of carriers. Thus, a long-lifetime light-emitting elementwhich does not easily deteriorate over time can be obtained.

That is, carrier balance changes less easily than the case where thecarrier balance is controlled with a single compound. For example, whenthe movement of carriers is controlled by a layer made of a singlecompound, the balance of the whole layer is changed by a partial changein morphology or by partial crystallization. Therefore, such alight-emitting element will easily deteriorate over time. However, asshown in this embodiment mode, when the movement of carriers iscontrolled with a component having the lowest weight percent of all thecomponents contained in the layer for controlling the movement ofcarriers, it is possible to reduce the effects of morphological change,crystallization, aggregation, or the like, whereby deterioration overtime can be suppressed. Therefore, a long-lifetime light-emittingelement whose luminous efficiency will not easily decrease over time canbe obtained.

As shown in this embodiment mode, a structure in which the layer forcontrolling the movement of carriers is provided between thelight-emitting layer and the second electrode functioning as the cathodeis particularly effective for a light-emitting element having excessiveelectrons. For example, the structure shown in this embodiment mode isparticularly effective for the case where the light-emitting layer hasan electron transporting property and the proportion of electronsinjected from the second electrode which penetrate the light-emittinglayer possibly increases over time. Note that this embodiment mode canbe combined with other embodiment modes as appropriate.

Embodiment Mode 2

This embodiment mode will describe one example of a light-emittingelement of the invention which differs from that shown in EmbodimentMode 1, with reference to FIG. 5A. This embodiment mode illustrates alight-emitting element which includes a layer for controlling themovement of holes as a layer for controlling the movement of carriers.The light-emitting element of the invention has a plurality of layersbetween a pair of electrodes. The plurality of layers is stacked bycombining layers made of a compound with a high carrier injectionproperty and a compound with a high carrier transporting property sothat a light-emitting region is formed at a position away from theelectrodes, i.e., so that carriers are recombined at a position awayfrom the electrodes.

In this embodiment mode, a light-emitting element includes a firstelectrode 402, a second electrode 404, and an EL layer 403 providedbetween the first electrode 402 and the second electrode 404. Note thatin this embodiment mode, description will be made on the assumption thatthe first electrode 402 functions as an anode and the second electrode404 functions as a cathode. That is, light emission is obtained when avoltage is applied to the first electrode 402 and the second electrode404 so that the potential of the first electrode 402 is higher than thepotential of the second electrode 404.

The substrate 401 can be similar to that described in Embodiment Mode 1.The first electrode 402 is preferably formed using a material with ahigh work function (i.e., 4.0 eV or higher) such as metals, alloys,electrically conductive compounds, or a mixture of them. Thus, amaterial similar to that described in Embodiment Mode 1 can be used.

The EL layer 403 includes a first layer 411, a second layer 412, a thirdlayer 413, a fourth layer 414, a fifth layer 415, and a sixth layer 416.Note that it is acceptable as long as the EL layer 403 includes a layerfor controlling the movement of carriers and a light-emitting layer thatare shown in this embodiment mode. Thus, the structure of the otherstacked layers is not specifically limited. For example, the EL layer403 can be formed by appropriate combination of a hole injection layer,a hole transporting layer, a light-emitting layer, a layer forcontrolling the movement of carriers, an electron transporting layer, anelectron injection layer, and the like.

The first layer 411 is a layer containing a compound with a high holeinjection property, and a material similar to that described inEmbodiment Mode 1 can be used. The second layer 412 is a layercontaining a compound with a high hole transporting property, and amaterial similar to that described in Embodiment Mode 1 can be used.

The third layer 413 is a layer for controlling the movement of carriers.The third layer 413 contains at least two kinds of compounds. In thethird layer 413, the weight percent of the first organic compound ishigher than that of the second organic compound. This embodiment modewill describe the case where the layer for controlling the movement ofcarriers is provided at a position closer to the first electrodefunctioning as the anode than the light-emitting layer is. That is, thecase where the layer for controlling the movement of carriers isprovided between the fourth layer 414 having the light-emitting functionand the first electrode 402 will be described.

When the layer for controlling the movement of carriers is provided at aposition closer to the first electrode functioning as the anode than thelight-emitting layer is, the first organic compound is preferably anorganic compound having a hole transporting property. That is, the firstorganic compound is preferably a compound whose hole transportingproperty is higher than the electron transporting property. In addition,the second organic compound is preferably an organic compound having afunction of trapping holes. That is, the second organic compound ispreferably an organic compound whose highest occupied molecular orbital(HOMO) level is higher than that of the first organic compound by 0.3 eVor more. When the layer for controlling the movement of carriersincludes the second organic compound, the electron transporting speed ofthe layer as a whole can be lower as compared with the case where thelayer is made of only the first organic compound. That is, adding thesecond organic compound makes it possible to control the movement ofcarriers. Further, controlling the concentration of the second organiccompound makes it possible to control the movement speed of carriers.The concentration of the second organic compound is preferably in therange of 0.1 wt % to 5 wt % or in the rage of 0.1 mol % to 5 mol %.

Examples of the second organic compound include CuPC, DNTPD, DPAB,bis[2-phenylpyridinato-N,C^(2′)]iridium(III) acetylacetonate(abbreviation: (abbreviation: Ir(ppy)₂acac),(acetylacetonato)bis[10-(2-pyridyl)phenoxazinato]iridium(III)(abbreviation: Ir(ppx)₂(acac)),tris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate (abbreviation: Ir(btp)₂(acac)),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine](abbreviation: DPABPA), and the like.

The above-described compounds are compounds having particularly highHOMO levels among compounds that are used for light-emitting elements.Thus, when such compounds are added to the first organic compound whichwill be described later, an excellent hole trapping property can beobtained. The second organic compound may emit light. In that case, itis preferable that the emission colors of the light-emitting layer andthe second organic compound be similar colors in order to keep the colorpurity of the light-emitting element.

In addition, the first organic compound contained in the third layer 413is an organic compound having a hole transporting property. That is, thefirst organic compound is a compound whose hole transporting property ishigher than the electron transporting property. Specifically, condensedaromatic hydrocarbons such as 9,10-diphenylanthracene (abbreviation:DPAnth) and 6,12-dimethoxy-5,11-diphenylchrysene can be used.

Alternatively, the following aromatic amine compounds can be used:N,N-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}9H-carbazol-3-amine(abbreviation: PCAPBA),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), NPB (or α-NPD), TPD, DFLDPBi, BSPB, and2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ). Further, high molecular compounds such as s PVK,PVTPA, PTPDMA, and Poly-TPD can also be used.

Above all, aromatic amine compounds that are stable against holes arepreferably used. In addition, as mentioned earlier, in Embodiment Mode2, the second organic compound should be a compound having a holetrapping property. Therefore, the HOMO level of the second organiccompound is preferably higher than that of the first organic compound by0.3 eV or more. Therefore, it is acceptable as long as the first organiccompound is appropriately selected so as to satisfy the above conditionaccording to the kind of compound used for the second organic compound.

Note that it is preferable that the emission colors of the highlylight-emissive compound contained in the fourth layer 414 and the secondorganic compound contained in the third layer 413 be similar colors.Therefore, it is preferable that a difference in peak values between theemission spectrum of the highly light-emissive compound and the emissionspectrum of the second organic compound be within the range of 30 nm.When the difference in peak values is within the range of 30 nm, theemission colors of the highly light-emissive compound and the secondorganic compound can be similar colors. Therefore, even when the secondorganic compound unintendedly emits light due to changes in voltage orthe like, changes in emission color can be suppressed.

When the highly light-emissive compound has higher luminous efficiencythan the second organic compound, it is preferable to control theconcentration of the second organic compound in the third layer 413 sothat only the light emission of the highly light-emissive compound canbe substantially obtained (by setting the concentration of the secondorganic compound to be slightly lower than that of the highlylight-emissive compound so that the light emission of the second organiccompound can be suppressed). In that case, the emission colors of thehighly light-emissive compound and the second organic compound aresimilar colors (i.e., they have about the same level of energy gap).Therefore, there is little possibility that energy will transfer fromthe highly light-emissive compound toward the second organic compound,and thus high luminous efficiency can be obtained.

FIG. 8 exemplarily illustrates a band diagram of the light-emittingelement of the invention shown in FIG. 5A. In FIG. 8, electrons injectedfrom the second electrode 404 pass through the sixth layer 416containing a compound with a high electron injection property andfurther through the fifth layer 415 containing a compound with a highelectron transporting property. Then, the electrons are injected to thefourth layer 414 containing a highly light-emissive compound. On theother hand, holes injected form the first electrode 402 pass through thefirst layer 411 containing a compound with a high hole injectionproperty and further through the second layer 412 containing a compoundwith a high hole transporting property. Then, the holes are injected tothe third layer 413 that is the layer for controlling the movement ofcarriers. The movement speed of the holes injected to the layer forcontrolling the movement of carriers is retarded by the second organiccompound having a hole trapping property. The holes whose movement speedhas been retarded are injected to the fourth layer 414 containing ahighly light-emissive compound, and then recombined with holes. Thus,light emission is obtained.

When the fourth layer 414 has a hole transporting property, for example,the movement speed of the electrons that are injected form the fifthlayer 145 to the fourth layer 414 is retarded. In addition, the movementspeed of the holes that are injected from the third layer 413 to thefourth layer 414 is even slow in the fourth layer 414 because it hasalready been retarded in the third layer 413. Therefore, holes of a slowmovement speed and electrons of a slow movement speed are recombined inthe fourth layer 414, whereby the recombination probability is increasedand luminous efficiency is improved.

In the case of a conventional light-emitting element which does notinclude the third layer 413, the movement speed of holes is not retardedbut the holes are directly injected to the fourth layer 414. Thus, theholes reach the vicinity of the interface between the fourth layer 414and the fifth layer 415. In that case, there is a possibility that theholes may reach and deteriorate the fifth layer 415. Further, when theamount of holes that have reached the fifth layer 415 is increased overtime, the recombination probability in the light-emitting layer isdecreased over time, which leads to a shorter lifetime of the element(luminance decay over time).

In the light-emitting element of the invention, holes injected from thefirst electrode 402 pass through the first layer 411 containing acompound with a high hole injection property and further through thesecond layer 412 containing a compound with a high hole transportingproperty. Then, the holes are injected to the third layer 413 that isthe layer for controlling the movement of carriers. Here, the thirdlayer 413 has a structure in which the second organic compound having afunction of trapping holes is added to the first organic compound havinga hole transporting property. Therefore, the movement speed of the holesthat are injected to the third layer 413 is retarded and the holeinjection to the fourth layer 414 is controlled.

As a result, a light-emitting region, which has conventionally beenformed in the vicinity of the interface between the fifth layer 415containing a compound with a high electron transporting property and thefourth layer 414, is formed around a region from the fourth layer 414 tothe vicinity of the interface between the fourth layer 414 and the thirdlayer 413. Therefore, there is low possibility that holes may reach anddeteriorate the fifth layer 415 which contains a compound with a highelectron transporting property. Similarly, as for electrons, there isalso low possibility that electrons may reach and deteriorate the secondlayer 412 which contains a compound with a high hole transportingproperty because the third layer 413 contains the first organic compoundhaving a hole transporting property.

Further, it is an important point of the invention that not merely acompound with low hole mobility is applied to the third layer 413 but anorganic compound having an a function of trapping holes is added to anorganic compound having a hole transporting property. With such astructure, it becomes possible not only to control the hole injection tothe fourth layer 414 but also to suppress changes in the controlledamount of hole injection over time. Therefore, the light-emittingelement of the invention can prevent a phenomenon that carrier balanceis lost over time, which could otherwise lower the recombinationprobability. Thus, the lifetime of the element can be improved(luminance decay over time can be suppressed).

In the light-emitting element of the invention, the light-emittingregion is not formed at the interface between the light-emitting layerand the hole transporting layer or the interface between thelight-emitting layer and the electron transporting layer. Therefore,there is no adverse effect of deterioration which would otherwise becaused if the light-emitting region is positioned close to the holetransporting layer or the electron transporting layer. Further, changesin carrier balance over time (in particular, changes in amount ofelectron injection over time) can be suppressed. Therefore, along-lifetime light-emitting element which does not easily deterioratecan be obtained.

In addition, it is preferable that the emission colors of the secondorganic compound contained in the third layer 413 and the highlylight-emissive compound contained in the fourth layer 414 be similarcolors. Specifically, it is preferable that a difference in peak valuesbetween the emission spectrum of the second organic compound and thehighly light-emissive compound be within the range of 30 nm. When thedifference in peak values is within the range of 30 nm, the emissioncolors of the second organic compound and the highly light-emissivecompound can be similar colors. Therefore, even when the second organiccompound unintendedly emits light due to changes in voltage or the like,changes in emission color can be suppressed.

In addition, the thickness of the third layer 413 that is the layer forcontrolling the movement of carriers is preferably in the range of 5 to20 nm. When the third layer 413 is too thick, the movement speed ofcarriers becomes too slow, which could result in high driving voltage.When the third layer 413 is too thin, on the other hand, it isimpossible to implement the function of controlling the movement ofcarriers. Therefore, the thickness of the third layer 413 is preferablyin the range of 5 to 20 nm. The fourth layer 414 is a layer containing ahighly light-emissive compound, and a compound similar to that describedin Embodiment Mode 1 can be used. Further, a phosphorescent materialsuch as(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)) can also be used.

When a phosphorescent material is used as the highly light-emissivecompound, the second organic compound contained in the third layer 413is also preferably a phosphorescent material such as Ir(ppy)₂(acac),Ir(ppx)₂(acac), Ir(ppy)₃, or Ir(btp)₂(acac). For example, whenIr(Fdpq)₂(acac) which exhibits red light emission is used for the highlylight-emissive compound, it is preferable to use Ir(btp)₂(acac) whichexhibits light emission of a similar red color for the second organiccompound contained in the third layer 413. Further, the light-emittinglayer may also have a structure in which a highly light-emissivecompound is dispersed in another compound as described in EmbodimentMode 1.

Note that in this embodiment mode, the layer for controlling themovement of carriers is provided between the light-emitting layer andthe first electrode functioning as the anode. Therefore, thelight-emitting layer preferably has a hole transporting property. Thatis, the hole transporting property of the light-emitting layer ispreferably higher than the electron transporting property thereof.Conventionally, when a light emitting layer has a hole transportingproperty, a hole blocking layer has been provided between thelight-emitting layer and a cathode in order to prevent holes frompenetrating the light-emitting layer. However, when the hole blockinglayer has deteriorated over time, a recombination region expands to theinside of the hoe blocking layer (or inside of the electron transportinglayer), which could result in a significant decrease in currentefficiency (i.e., luminance decay). Meanwhile, in the invention, themovement of holes is controlled before the holes reach thelight-emitting layer (between the light-emitting layer and the anode).Therefore, even when the balance of holes (e.g., mobility or the amountof holes relative to that of electrons) is somewhat lost, the proportionof recombination in the light-emitting layer hardly changes, which isadvantageous in that luminance does not easily decay.

Therefore, it is preferable to use an organic compound with a holetransporting property as the material in which the highly light-emissivecompound is dispersed that is described in Embodiment Mode 1.Specifically, the following can be used: condensed aromatic hydrocarbonssuch as DPAnth and 6,12-dimethoxy-5,11-diphenylchrysene, or aromaticamine compounds such as CzA1PAA, DPhPA, PCAPA, PCAPBa, 2PCAPA, NPB (orα-NPD), TPD, DFLDPBi, and BSPB.

The fifth layer 415 is a layer containing a compound with a highelectron transporting property, and a compound similar to that describedin Embodiment Mode 1 can be used. The sixth layer 416 is a layercontaining a compound with a high electron injection property, and acompound similar to that described in Embodiment Mode 1 can be used.

As a method forming the EL layer, various methods can be used regardlessof a dry process or a wet process. For example, a vacuum depositionmethod, an ink-jet method, a spin coating method, or the like can beused. Further, different deposition methods may be used for differentelectrodes or different layers. For example, among the above-describedmaterials, a high molecular compound may be selected to form the ELlayer by a wet process. Alternatively, a low molecular organic compoundmay be selected to form the EL layer by a wet process. Further, it isalso possible to form the EL layer by selecting a low molecular organiccompound and using a dry process such as a vacuum deposition method.Similarly, the electrodes can be formed by a wet process such as asol-gel process or by a wet process with a paste of a metal material.Alternatively, the electrodes can be formed by a dry process such as asputtering method or a vapor deposition method.

A specific method for forming the light-emitting element will bedescribed below. In the case where the light-emitting element of theinvention is applied to a display device and its light-emitting layer isselectively deposited according to each color, the light-emitting layeris preferably formed by a wet process. When the light-emitting layer isformed by an ink-jet method, selective deposition of the light-emittinglayer for each color can be easily performed even when a large substrateis used.

For example, the structure shown in FIG. 5A can be obtained by the stepsof: forming the first electrode by a sputtering method which is a dryprocess, forming the first layer by an ink-jet method or a spin coatingmethod which is a wet process, forming the second layer by a vacuumdeposition method which is a dry process, forming the third layer by anink-jet method which is a wet process, forming the fourth layer by aco-deposition method which is a dry process, forming the fifth layer andthe sixth layer by a vacuum deposition method which is a dry process,and forming the second electrode by an ink-jet method or a spin coatingmethod which is a wet process.

Alternatively, the structure shown in FIG. 5A may be obtained by thesteps of: forming the first electrode by a ink-jet method which is a wetprocess, forming the first layer by a vacuum deposition method which isa dry process, forming the second layer by an ink-jet method or a spincoating method which is a wet process, forming the third layer by anink-jet method which is a wet process, forming the fourth layer by anink-jet method or a spin coating method which is a wet process, formingthe fifth layer and the sixth layer by an ink-jet method or a spincoating method which is a wet process, and forming the second electrodeby an ink-jet method or a spin coating method which is a wet process.Note that the deposition methods are not limited to the above methods,and a wet process and a dry process may be combined as appropriate.

To be more specific, the structure shown in FIG. 5A can be obtained bythe steps of: forming the first electrode by a sputtering method whichis a dry process, forming the first layer and the second layer by avacuum deposition method which is a dry process, forming the third layerby a co-deposition method which is a dry process, forming the fourthlayer which is a light-emitting layer by an ink-jet method which is awet process, forming the fifth layer by an ink-jet method or a spincoating method which is a wet process, not forming the sixth layer, andforming the second electrode by an ink-jet method or a spin coatingmethod which is a wet process.

That is, the first electrode to the third layer can be formed by dryprocesses, while the fourth layer to the second electrode can be formedby wet processes. By this method, the first electrode to the third layercan be consecutively formed in vacuum, and the fourth layer to thesecond electrode can be formed at atmospheric pressure. In addition,selective deposition of the fourth layer for each color can be easilyperformed. Therefore, the process can be simplified and productivity canbe improved.

Meanwhile, a structure shown in FIG. 6A can be formed in the reverseorder of the above-described steps: forming the second electrode by anink-jet method or a spin coating method which is a wet process, formingthe sixth layer and the fifth layer by an ink-jet method or a spincoating method which is a wet process, forming the fourth layer by anink-jet method which is a wet process, forming the third layer by aco-deposition method which is a dry process, forming the second layerand the first layer by a vacuum deposition method which is a dryprocess, and forming the first electrode by a vacuum deposition methodwhich is a dry process. By this method, the second electrode to thefourth layer can be formed at atmospheric pressure, and the third layerto the first electrode can be consecutively formed in vacuum by dryprocesses. Therefore, the process can be simplified and productivity canbe improved.

In the light-emitting element of the invention having the abovestructure, a current flows due to a potential difference generatedbetween the first electrode 402 and the second electrode 404, wherebyholes and electrons are recombined in the EL layer 403 and lightemission is obtained. Light emission is extracted outside through one orboth of the first electrode 402 and the second electrode 404. Therefore,one or both of the first electrode 402 and the second electrode 404 is alight-transmissive electrode.

When only the first electrode 402 is a light-transmissive electrode,light emission is extracted from the substrate side through the firstelectrode 402 as shown in FIG. 7A. Meanwhile, when only the secondelectrode 404 is a light-transmissive electrode, light emission isextracted from a side opposite to the substrate side through the secondelectrode 404 as shown in FIG. 7B. When both of the first electrode 402and the second electrode 404 are light-transmissive electrodes, lightemission is extracted from both the substrate side and the side oppositeto the substrate side through the first electrode 402 and the secondelectrode 404 as shown in FIG. 7C.

Note that the structure of the layers provided between the firstelectrode 402 and the second electrode 404 is not limited to the abovestructure. That is, in the invention and in this embodiment mode, anystructure other than the above structure may be employed as long as alight-emitting region for recombination of holes and electrons ispositioned away from the first electrode 402 and the second electrode404, and also a layer for controlling the movement of carriers isprovided so as to prevent quenching which would otherwise be caused bythe proximity of the light-emitting region to metal.

That is, the stacked structure of the layers is not particularlylimited. It is acceptable as long as layers made of a compound with ahigh electron transporting property, a compound with a high holetransporting property, a compound with a high electron injectionproperty, a compound with a high hole injection property, and a compoundwith a bipolar property (a compound having both high electron and holetransporting properties) are appropriately combined with the layer forcontrolling the movement of carriers and the light-emitting layer thatare shown in this embodiment mode.

Note that the layer for controlling the movement of carriers shown inthis embodiment mode is a layer for controlling the movement of holes.Therefore, it is preferably provided at a position closer to theelectrode functioning as the anode than the light-emitting layer is. Forexample, as shown in FIG. 5B, a seventh layer 417 containing a compoundwith a high hole transporting property may be provided between thefourth layer 414 having the light-emitting function and the third layer413 that is the layer for controlling the movement of carriers.

More preferably, the layer for controlling the movement of carriers isdesirably provided to be in contact with the light-emitting layer. Whenthe layer for controlling the movement of carriers is provided to be incontact with the light-emitting layer, hole injection to thelight-emitting layer can be directly controlled. Therefore, changes incarrier balance over time in the light-emitting layer can be controlledmore efficiently, whereby the lifetime of the element can be moreeffectively prolonged. In addition, since the seventh layer containing acompound with a high hole transporting property is not required, theprocess can be simplified.

Note that when the layer for controlling the movement of carriers isprovided to be in contact with the light-emitting layer, it ispreferable that the first organic compound contained in the layer forcontrolling the movement of carriers be different from an organiccompound which is contained in large quantities in the light-emittinglayer. In particular, when the light-emitting layer contains a compound(a third organic compound) in which a highly light-emissive compound isdispersed and a highly light-emissive compound (a fourth organiccompound), it is desirable that the third organic compound and the firstorganic compound be different organic compounds. With such a structure,the movement of carriers (in this embodiment mode, holes) can becontrolled even between the first organic compound and the third organiccompound. Thus, the advantageous effect of providing the layer forcontrolling the movement of carriers can be further increased.

In addition, the light-emitting element shown in FIG. 6A has a structurein which the second electrode 404 functioning as the cathode, the ELlayer 203, and the first electrode 402 functioning as the anode aresequentially stacked over the substrate 401. The EL layer 403 includesthe first layer 411, the second layer 412, the third layer 413, thefourth layer 414, the fifth layer 415, and the sixth layer 416. Thethird layer 413 is provided at a position closer to the first electrodefunctioning as the anode than the fourth layer 414 is.

The light-emitting element of the invention includes a layer forcontrolling the movement of carriers. The layer for controlling themovement of carriers contains at least two kinds of compounds.Therefore, by controlling the compatibility of compounds, the mixtureratio thereof, the thickness of the layer, and the like, carrier balancecan be precisely controlled. Further, since the carrier balance can becontrolled by controlling the compatibility of compounds, the mixtureratio thereof, the thickness of the layer, and the like, carrier balancecan be more easily controlled than in a conventional light-emittingelement. That is, the movement of carriers can be controlled not bychanging the physical properties of the material but by controlling themixture ratio, the thickness of the layer, and the like.

When the carrier balance is improved by using the layer for controllingthe movement of carriers, the luminous efficiency of the light-emittingelement can be improved. Further, using the layer for controlling themovement of the carriers makes it possible to prevent excessive holesfrom being injected and also prevent holes from penetrating thelight-emitting layer to reach the electron transporting layer or theelectron injection layer. When holes have reached the electrontransporting layer or the electron injection layer, the recombinationprobability in the light-emitting layer decreases (i.e., carrier balanceis lost), which could result in the decrease in luminous efficiency overtime. That is, the lifetime of the light-emitting element becomesshorter.

However, by using the layer for controlling the movement of carriers asshown in this embodiment mode, it becomes possible to prevent excessiveholes from being injected and also prevent holes from penetrating thelight-emitting layer to reach the electron transporting layer or thehole injection layer. Therefore, a decrease in luminous efficiency overtime can be suppressed. That is, a long-lifetime light-emitting elementcan be obtained.

To be more specific, between the two or more kinds of compoundscontained in the layer for controlling the movement of carriers, thesecond organic compound which has a lower weight percent than the weightpercent of the first organic compound is used for controlling themovement of carriers. Therefore, the movement of carriers can becontrolled with a component having the lowest weight percent of all ofthe components contained in the layer for controlling the movement ofcarriers. Thus, a long-lifetime light-emitting element which does noteasily deteriorate over time can be obtained.

For example, when the movement of carriers is controlled by a layer madeof a single compound, the balance of the whole layer is changed by apartial change in morphology or by partial crystallization. Therefore,such a light-emitting element will easily deteriorate over time.However, as shown in this embodiment mode, when the movement of carriersis controlled with a component having the lowest weight percent of allthe components contained in the layer for controlling the movement ofcarriers, it is possible to reduce the effects of morphological change,crystallization, aggregation, or the like, whereby deterioration overtime can be suppressed. Therefore, a long-lifetime light-emittingelement whose luminous efficiency will not easily decrease over time canbe obtained.

As shown in this embodiment mode, a structure in which the layer forcontrolling the movement of carriers is provided between thelight-emitting layer and the first electrode functioning as the anode isparticularly effective for a light-emitting element having excessiveholes. For example, the structure shown in this embodiment mode isparticularly effective for the case where the light-emitting layer has ahole transporting property and the proportion of holes injected from thefirst electrode which penetrate the light-emitting layer possiblyincreases over time.

Note that this embodiment mode can be combined with other embodimentmodes as appropriate. For example, a layer for controlling the movementof holes may be provided between the light-emitting layer and the firstelectrode functioning as the anode, and also a layer for controlling themovement of electrons may be provided between the light-emitting layerand the second electrode functioning as the cathode. That is, when sucha structure is employed, it becomes possible to provide layers forcontrolling the movement of carriers on opposite sides of thelight-emitting layer, which is more preferable in that carriers can berecombined on opposite sides of the light-emitting layers at positionsaway from the electrodes. As a result, the movement of carriers iscontrolled on opposite sides of the light-emitting layer, whereby theeffects of morphological change, crystallization, aggregation, or thelike can be further reduced, and deterioration over time can besuppressed. Therefore, a long-lifetime light-emitting element whoseluminous efficiency will not easily decrease over time can be obtained.

Embodiment Mode 3

This embodiment mode will describe a light-emitting element in which aplurality of light-emitting units in accordance with the invention isstacked, with reference to FIG. 9. The light-emitting element is astacked-type light-emitting element which has a plurality oflight-emitting units between a first electrode and a second electrode. Astructure similar to that of the EL layer 203 shown in Embodiment Mode 1or the EL layer 403 shown in Embodiment Mode 2 can be used for eachlight-emitting unit. In other words, the light-emitting elementdescribed in each of Embodiment Modes 1 and 2 is a light-emittingelement having a single light-emitting unit. In this embodiment mode, alight-emitting element having a plurality of light-emitting units willbe described.

In FIG. 9, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502. A charge generation layer 513 is provided between thefirst light-emitting unit 511 and the second light-emitting unit 512. Anelectrode similar to that shown in Embodiment Mode 1 can be applied tothe first electrode 501 and the second electrode 502. The firstlight-emitting unit 511 and the second light-emitting unit 512 may haveeither the same structure or different structures, and a structuresimilar to those shown in Embodiment Modes 1 and 2 can be applied.

The charge generation layer 513 contains a composite material of anorganic compound and metal oxide. The composite material of an organiccompound and metal oxide is described in Embodiment Mode 1, and includesan organic compound and metal oxide such as vanadium oxide, molybdenumoxide, or tungsten oxide. As the organic compound, various compoundssuch as aromatic amine compounds, carbazole derivatives, aromatichydrocarbons, and high molecular compounds (e.g., oligomer, dendrimer,or polymer) can be used. It is preferable to use an organic compoundhaving a hole mobility of greater than or equal to 10⁻⁶ cm²/Vs as theorganic compound having a hole transporting property. However, othercompounds than these compounds may also be used as long as the holetransporting properties thereof are higher than the electrontransporting properties thereof. The composite material of an organiccompound and metal oxide is superior in carrier injection property andcarrier transporting property, and therefore, low-voltage driving andlow-current driving can be realized.

Note that the charge generation layer 513 may be formed with acombination of a layer containing a composite material of an organiccompound and metal oxide and other materials. For example, the chargegeneration layer 513 may be formed with a combination of a layercontaining the composite material of an organic compound and metal oxideand a layer containing one compound selected from electron donatingcompounds and a compound having a high electron transporting property.Further, the charge generation layer 513 may be formed with acombination of a layer containing the composite material of an organiccompound and metal oxide and a transparent conductive film.

In any case, the charge generation layer 513 sandwiched between thefirst light-emitting unit 511 and the second light-emitting unit 512 isacceptable as long as electrons are injected to one light-emitting unitand holes are injected to the other light-emitting unit when a voltageis applied between the first electrode 501 and the second electrode 502.For example, when a voltage is applied so that a potential of the firstelectrode is higher than a potential of the second electrode, anystructure is acceptable for the charge generation layer 513 as long asthe layer 513 injects electrons and holes into the first light-emittingunit 511 and the second light-emitting unit 512, respectively.

Although this embodiment mode illustrates the light-emitting elementhaving two light-emitting units, the invention can be similarly appliedto a light-emitting element in which three or more light-emitting unitsare stacked. By arranging a plurality of light-emitting units between apair of electrodes in such a manner that the plurality of light-emittingunits is partitioned with a charge generation layer, a light-emittingelement having a long life and high luminance can be realized whilemaintaining a low current density. In addition, when the light-emittingelement is applied to lighting, a voltage drop which would be caused bythe resistance of the electrode materials can be suppressed. Thus,uniform light emission over a large area becomes possible. In otherwords, a light-emitting device capable of low-voltage driving andlow-power consumption can be realized.

When the light-emitting units are formed to have different emissioncolors from each other, light emission of a desired color can beobtained as a whole from the light-emitting element. For example, in thelight-emitting element having two light-emitting units, when theemission color of the first light-emitting unit and the emission colorof the second light-emitting unit are complementary colors, alight-emitting element which emits white light as a whole can beobtained. Note that “complementary colors” refer to colors which canproduce an achromatic color when mixed. That is, when light emitted fromcompounds which emit light of complementary colors are mixed, whiteemission can be obtained. The same can be said for a light-emittingelement which has three light-emitting units. For example, whiteemission as a whole can be obtained from the light-emitting element whenthe emission color of the first light-emitting unit is red, the emissioncolor of the second light-emitting unit is green, and the emission colorof the third light-emitting unit is blue. Note that this embodiment modecan be combined with other embodiment modes as appropriate.

Embodiment Mode 4

This embodiment mode will describe a light-emitting device having thelight-emitting element of the invention. In this embodiment mode, alight-emitting device having a pixel portion which includes thelight-emitting element of the invention will be described with referenceto FIGS. 10A and 10B. FIG. 10A is a top view of a light-emitting device,and FIG. 10B is a cross-sectional view taken along lines A-A′ and B-B′of FIG. 4A. Reference numerals 601, 602, and 603 denote a driver circuitportion (a source driver circuit), a pixel portion, and a driver circuitportion (a gate driver circuit), respectively, which are indicated bydotted lines. In addition, reference numerals 604 and 605 denote asealing substrate and a sealing material, respectively, and a portionsurrounded by the sealing material 605 corresponds to a space 607.

A lead wiring 608 is a wiring for transmitting signals to the sourcedriver circuit 601 and the gate driver circuit 603, and this wiring 608receives video signals, clock signals, start signals, reset signals, andthe like from an FPC (Flexible Printed Circuit) 609 that is an externalinput terminal. Although only the FPC is shown here, the FPC may beprovided with a printed wiring board (PWB). The light-emitting device inthis specification includes not only a light-emitting device itself butalso a light-emitting device with an FPC or a PWB attached thereto.

Next, a cross-sectional structure will be described with reference toFIG. 10B. The driver circuit portion and the pixel portion are formedover a substrate 610. Here, the source driver circuit 601, which is thedriver circuit portion, and one pixel in the pixel portion 602 areshown. A CMOS circuit, which is a combination of an n-channel TFT 623and a p-channel TFT 624, is formed as the source driver circuit 601. Thedriver circuit may be formed using various CMOS circuits, PMOS circuits,or NMOS circuits. Although a driver-integration type device, in which adriver circuit is formed over the same substrate as a pixel portion, isshown in this embodiment mode, a driver circuit is not necessarilyformed over the same substrate as a pixel portion and can be formedoutside the substrate.

The pixel portion 602 has a plurality of pixels, each of which includesa switching TFT 611, a current-controlling TFT 612, and a firstelectrode 613 which is electrically connected to a drain of thecurrent-controlling TFT 612. Note that an insulator 614 is formed so asto cover an end portion of the first electrode 613. Here, a positivephotosensitive acrylic resin film is used for the insulator 614.

The insulator 614 is formed so as to have a curved surface havingcurvature at an upper end portion or a lower end portion thereof inorder to obtain favorable coverage. For example, in the case of using apositive photosensitive acrylic resin as a material for the insulator614, the insulator 614 is preferably formed so as to have a curvedsurface with a curvature radius (0.2 to 3 μm) only at the upper endportion thereof. Either a negative photoresist which becomes insolublein an etchant by light irradiation or a positive photoresist whichbecomes soluble in an etchant by light irradiation can be used for theinsulator 614.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. Various metals, alloys, electrically conductivecompounds, or mixture of them can be used for a material for forming thefirst electrode 613. When the first electrode 613 is used as an anode,it is particularly preferable to select a material with a high workfunction (a work function of 4.0 eV or higher) among such metals,alloys, electrically conductive compounds, and mixture of them.

For example, the first electrode 613 can be formed by using asingle-layer film such as a film made of ITO containing silicon, a filmmade of indium zinc oxide (IZO), a titanium nitride film, chromium film,a tungsten film, a Zn film, or a Pt film; stacked layers of a titaniumnitride film and a film containing aluminum as its main component; athree-layer structure of a titanium nitride film, a film containingaluminum as its main component, and a titanium nitride film; or thelike. When the first electrode 613 has a stacked structure, theelectrode 613 shows low resistance enough to serve as a wiring, giving agood ohmic contact. Further, the first electrode 613 can function as ananode.

In addition, the EL layer 616 is formed by various methods such as avapor-deposition method using an evaporation mask, an ink-jet method,and a spin coating method. The EL layer 616 includes the layer forcontrolling the movement of carriers and the light-emitting layer thatare described in Embodiment Modes 1 and 2. As other materials forforming the EL layer 616, low molecular compounds or high molecularcompounds (e.g., oligomer or dendrimer) may also be used. In addition,not only organic compounds, but also inorganic compounds can be used forthe material for forming the EL layer.

As a material for forming the second electrode 617, various metals,alloys, electrically conductive compounds, or mixture of them can beused. When the second electrode 617 is used as a cathode, it isparticularly preferable to select a material with a low work function (awork function of 3.8 eV or lower) among such metals, alloys,electrically conductive compounds, and mixture of them. For example,elements belonging to Group 1 or 2 of the periodic table, i.e., alkalimetals such a lithium (Li) and cesium (Cs) and alkaline earth metalssuch as magnesium (Mg), calcium (Ca), and strontium (Sr); alloys of them(e.g., MgAg and AlLi); and the like can be used.

In the case where light generated in the EL layer 616 is transmittedthrough the second electrode 617, the second electrode 617 may also beformed using stacked layers of a thin metal film and a transparentconductive film (e.g., indium tin oxide (ITO), ITO containing silicon orsilicon oxide, indium zinc oxide (IZO), or indium oxide containingtungsten oxide and zinc oxide (IWZO)).

In the light-emitting device of this embodiment mode, the sealingsubstrate 604 is attached to the element substrate 610 with the sealingmaterial 605, whereby a light-emitting element 618 is provided in thespace 607 surrounded by the element substrate 610, the sealing substrate604, and the sealing material 605. Note that the space 607 is filledwith an inert gas (e.g., nitrogen or argon). There is also a case wherethe space 607 is filled with the sealing material 605.

Note that an epoxy resin is preferably used for the sealing material605. Such material preferably allows as little moisture and oxygen aspossible to penetrate. As a material for forming the sealing substrate604, a glass substrate or a quartz substrate can be used as well as aplastic substrate made of FRP (Fiberglass-Reinforced Plastics), PVF(polyvinyl fluoride), polyester, acrylic, or the like.

By the above-described process, a light-emitting device having thelight-emitting element of the invention can be obtained. The thuslyobtained light-emitting device of the invention has a long-lifetimelight-emitting element; therefore, the device itself also has a longlifetime. Further, since the light-emitting element of the invention isa light-emitting element with high luminous efficiency, high luminancecan be achieved, and a light-emitting device with reduced powerconsumption can be obtained.

Although this embodiment mode illustrates an active matrixlight-emitting device in which driving of a light-emitting element iscontrolled by a transistor, it is also possible to use a passive matrixlight-emitting device in which a light-emitting element is drivenwithout an element for driving the light-emitting element such as atransistor, like the structure shown in FIGS. 11A and 11B. FIGS. 11A and11B show a perspective view and a cross-sectional view, respectively, ofa passive matrix light-emitting device which is formed by applying theinvention. FIG. 11 A is a cross-sectional view of a light-emittingdevice, and FIG. 11B is a cross-sectional view taken along a line X-Y ofFIG. 11A. In FIGS. 11A and 11B, an EL layer 955 is provided between anelectrode 952 and an electrode 956 over a substrate 951. An edge of theelectrode 952 is covered with an insulating layer 953.

A partition layer 954 is provided over the insulating layer 953. A sidewall of the partition layer 954 slopes so that a distance between oneside wall and the other side wall becomes narrow toward a substratesurface. In other words, a cross section of the partition layer 954 inthe direction of a short side is trapezoidal, and a bottom base (a sideexpanding in the same direction as a plane direction of the insulatinglayer 953 and in contact with the insulating layer 953) is shorter thana top base (a side expanding in the same direction as the planedirection of the insulating layer 953 and not in contact with theinsulating layer 953). The partition layer 954 provided in this mannercan prevent the light-emitting element from being defective due tostatic electricity or the like. In addition, a passive matrixlight-emitting device can also be formed as a long-lifetimelight-emitting device by applying the long-lifetime light-emittingelement of the invention. Further, by applying the light-emittingelement of the invention which has high luminous efficiency, alight-emitting device with reduced power consumption can be obtained.

Embodiment Mode 5

This embodiment mode will describe an electronic device of the inventionwhich includes the light-emitting device described in Embodiment Mode 4as a component part. The electronic device of the invention includes thelight-emitting elements described in any of Embodiment Modes 1 to 3, andthus has a display portion with a long lifetime. In addition, since thelight-emitting elements with high luminous efficiency are used, adisplay portion with reduced power consumption can be obtained.

Typical examples of an electronic device which is formed using thelight-emitting device of the invention include cameras such as videocameras and a digital cameras, goggle displays, navigation systems,audio reproducing devices (e.g., car audio component stereos and audiocomponent stereos), computers, game machines, portable informationterminals (e.g., mobile computers, mobile phones, portable gamemachines, and electronic books), and image reproducing devices providedwith recording media (specifically, a device capable of reproducing thecontent of a recording medium such as a digital versatile disc (DVD) andprovided with a display device that can display the reproduced image),and the like. Specific examples of these electronic devices are shown inFIGS. 12A to 12D.

FIG. 12A shows a television set in accordance with the invention, whichincludes a housing 9101, a supporting base 9102, a display portion 9103,speaker portions 9104, video input terminals 9105, and the like. In thetelevision set, the display portion 9103 has a matrix arrangement oflight-emitting elements similar to those shown in Embodiment Modes 1 to3. The feature of the light-emitting elements is exemplified by the longlifetime. The display portion 9103 which includes the light-emittingelements has similar features. Therefore, this television set also has afeature of the long lifetime. That is, a television set which is durablefor use over a long period of time can be provided. Further, sincelight-emitting elements with high luminous efficiency are used, atelevision set having a display portion with reduced power consumptioncan be obtained.

FIG. 12B shows a computer in accordance with the invention, whichincludes a main body 9201, a housing 9202, a display portion 9203, akeyboard 9204, an external connection port 9205, a pointing device 9206,and the like. In the computer, the display portion 9203 has a matrixarrangement of light-emitting elements similar to those shown inEmbodiment Modes 1 to 3. The feature of the light-emitting elements isexemplified by the long lifetime. The display portion 9203 whichincludes the light-emitting elements has similar features. Therefore,this computer also has a feature of the long lifetime. That is, acomputer which is durable for use over a long period of time can beprovided. Further, since light-emitting elements with high luminousefficiency are used, a computer having a display portion with reducedpower consumption can be obtained.

FIG. 12C shows a mobile phone in accordance with the invention, whichincludes a main body 9401, a housing 9402, a display portion 9403, anaudio input portion 9404, an audio output portion 9405, operation keys9406, an external connection port 9407, an antenna 9408, and the like.In the mobile phone, the display portion 9403 has a matrix arrangementof light-emitting elements similar to those shown in Embodiment Modes 1to 3. The feature of the light-emitting elements is exemplified by thelong lifetime. The display portion 9403 which includes thelight-emitting elements has similar features. Therefore, this mobilephone also has a feature of the long lifetime. That is, a mobile phonewhich is durable for use over a long period of time can be provided.Further, since light-emitting elements with high luminous efficiency areused, a mobile phone having a display portion with reduced powerconsumption can be obtained.

FIG. 12D shows a camera in accordance with the invention, which includesa main body 9501, a display portion 9502, a housing 9503, an externalconnection port 9504, a remote control receiving portion 9505, an imagereceiving portion 9506, a battery 9507, an audio input portion 9508,operation keys 9509, an eye piece portion 9510, and the like. In thecamera, the display portion 9502 has a matrix arrangement oflight-emitting elements similar to those shown in Embodiment Modes 1 to3. The feature of the light-emitting elements is exemplified by the longlifetime. The display portion 9502 which includes the light-emittingelements has similar features. Therefore, this camera also has a featureof the long lifetime. That is, a camera which is durable for use over along period of time can be provided. Further, since light-emittingelements with high luminous efficiency are used, a camera having adisplay portion with reduced power consumption can be obtained.

As described above, the applicable range of the light-emitting device ofthe invention is so wide that the light-emitting device can be appliedto electronic devices in various fields. By using the light-emittingdevice of the invention, an electronic device having a long-lifetimedisplay portion which is durable for use over a long period of time canbe provided. In addition, an electronic device having a display portionwith reduced power consumption can be obtained.

The light-emitting device of the invention can also be used as alighting device. An example of using the light-emitting element of theinvention for a lighting device will be described with reference to FIG.13. FIG. 13 shows an example of a liquid crystal display device whichuses the light-emitting device of the invention as a backlight. Theliquid crystal display device shown in FIG. 13 includes a housing 901, aliquid crystal layer 902, a backlight 903, and a housing 904, and theliquid crystal layer 902 is connected to a driver IC 905. Thelight-emitting device of the invention is used for the backlight 903,and current is supplied through a terminal 906.

By using the light-emitting device of the invention as the backlight ofthe liquid crystal display device, a backlight with a long lifetime canbe obtained. The light-emitting device of the invention is a lightingdevice with plane light emission, and can have a large area. Therefore,the backlight can have a large area, and a liquid crystal display devicehaving a large area can be obtained. Furthermore, the light-emittingdevice of the invention has a thin shape and has low power consumption;therefore, a thin shape and low power consumption of a display devicecan also be achieved. In addition, since light-emitting elements withhigh luminous efficiency are used, a light-emitting device with highluminance can be obtained. Furthermore, since the light-emitting deviceof the invention has a long lifetime, a liquid crystal display devicehaving a long lifetime can be obtained.

FIG. 14 shows an example of using the light-emitting device of theinvention for a table lamp which is a lighting device. A table lampshown in FIG. 14 has a housing 2001 and a light source 2002, and thelight-emitting device of the invention is used as the light source 2002.The light-emitting device of the invention has a long lifetime;therefore, a table lamp also has a long lifetime. FIG. 15 shows anexample of using the light-emitting device of the invention for anindoor lighting device 3001. Since the light-emitting device of theinvention can have a large area, the light-emitting device of theinvention can be used as a lighting device having a large emission area.Furthermore, since the light-emitting device of the invention has a longlifetime, a lighting device having a long lifetime can be obtained.

When a television set 3002 in accordance with the invention like the oneillustrated in FIG. 12A is placed in a room in which the light-emittingdevice of the invention is used as the indoor lighting device 3001,public broadcasting and movies can be watched. In such a case, sinceboth of the devices have long lifetimes, frequency of replacement of thelighting device and the television set can be reduced, and damage on theenvironment can be reduced.

Embodiment 1

This embodiment will specifically describe a fabrication example of thelight-emitting element of the invention and the characteristics of thefabricated light-emitting element, with reference to the stackedstructure in FIG. 16. In addition, the characteristics will bespecifically described with reference to graphs showing the measurementresults. Structural formulae of organic compounds used in Embodiment 1are shown below.

(Fabrication of Light-Emitting Element 1)

First, ITO containing silicon oxide was deposited over a glass substrate2201 by a sputtering method, whereby a first electrode 2202 was formed.Note that the thickness of the first electrode 2202 was 110 nm and theelectrode area was 2 mm×2 mm. Next, the substrate having the firstelectrode 2202 was fixed to a substrate holder provided in a vacuumdeposition apparatus in such a way that the surface of the firstelectrode faced downward, and then the pressure was reduced to about10⁻⁴ Pa.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)and molybdenum(VI) oxide were co-deposited on the first electrode 2202,whereby a layer 2211 containing a composite material was formed. Thedeposition rate was controlled so that the thickness of the layer 2211could be 50 nm and the weight ratio of NPB to molybdenum(VI) oxide couldbe 4:1 (=NPB:molybdenum oxide). Note that the co-deposition method is adeposition method in which deposition is performed using a plurality ofevaporation sources at the same time in one treatment chamber.

Next, a hole transporting layer 2212 was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) to athickness of 10 nm by a deposition method using resistance heating.After that, a light-emitting layer 2213 was formed over the holetransporting layer 2212. The light-emitting layer 2213 was formed byco-depositing 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) to a thickness of 30 nm. Here, the depositionrate was controlled so that the weight ratio of CzPA to 2PCAPA could be1:0.05 (=CzPA:2PCAPA).

Further, a layer 2214 for controlling the movement of carriers wasformed by co-depositing tris(8-quinolinolato)aluminum(III)(abbreviation: Alq) and N,N′-diphenylquinacridon (abbreviation: DPQd) toa thickness of 10 nm over the light-emitting layer 2213. Here, thedeposition rate was controlled so that the weight ratio of Alq to DPQdcould be 1:0.003 (=Alq:DPQd).

Next, an electron transporting layer 2215 was formed by depositingtris(8-quinolinolato)aluminum(III) (abbreviation: Alq) to a thickness of30 nm over the layer 2214 for controlling the movement of carriers by adeposition method using resistance heating. After that, an electroninjection layer 2216 was formed by depositing lithium fluoride (LiF) toa thickness of 1 nm over the electron transporting layer 2215. Finally,a second electrode 2204 was formed by depositing aluminum to a thicknessof 200 nm by a deposition method using resistance heating. Consequently,a light-emitting element 1 was formed.

The light-emitting element 1 of the invention obtained through theabove-described process was put into a glove box containing a nitrogenatmosphere so that the light-emitting element was sealed fromatmospheric air. Then, the operating characteristics of thelight-emitting element 1 were measured. Note that the measurement wasconducted at room temperature (atmosphere kept at 25° C.). FIG. 17 showsthe current density vs. luminance characteristics of the light-emittingelement 1. FIG. 18 shows the voltage vs. luminance characteristics ofthe light-emitting element 1. FIG. 19 shows the luminance vs. currentefficiency characteristics of the light-emitting element 1. FIG. 20shows the emission spectrum of the light-emitting element 1 with acurrent supply of 1 mA.

FIG. 21 shows the result of a continuous lighting test in which thelight-emitting element 1 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² is100%). The emission color of the light-emitting element 1 was located atthe CIE chromaticity coordinates of (x=0.30, y=0.64) at the luminance of5000 cd/m², and green emission which derives from 2PCAPA was obtained.In addition, the current efficiency and driving voltage of thelight-emitting element 1 at the luminance of 5000 cd/m² were 14 cd/A and8.1 V, respectively. Further, when a continuous lighting test wasconducted in which the light-emitting element 1 was continuously lit byconstant current driving with the initial luminance set at 5000 cd/m²,93% of the initial luminance was maintained even after 740 hours. Thus,it was proved that the light-emitting element 1 has a long lifetime.

(Fabrication of Reference Light-Emitting Element 2)

Next, for the sake of comparison, a reference light-emitting element 2without the layer for controlling the movement of carriers that theabove-described light-emitting element 1 has was formed. The fabricationmethod will be described below. First, ITO containing silicon oxide wasdeposited over a glass substrate by a sputtering method, whereby a firstelectrode was formed. Note that the thickness of the first electrode was110 nm and the electrode area was 2 mm×2 mm.

Next, the substrate having the first electrode was fixed to a substrateholder provided in a vacuum deposition apparatus in such a way that thesurface of the first electrode faced downward, and then the pressure wasreduced to about 10⁻⁴ Pa. Then,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-deposited on the first electrode, whereby alayer containing a composite material was formed. The deposition ratewas controlled so that the thickness of the layer containing a compositematerial could be 50 nm and the weight ratio of NPB to molybdenum(VI)oxide could be 4:1 (=NPB:molybdenum oxide).

Next, a hole transporting layer was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) to athickness of 10 nm by a deposition method using resistance heating.After that, a light-emitting layer was formed over the hole transportinglayer. The light-emitting layer was formed by co-depositing9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) to a thickness of 40 mm. Here, the depositionrate was controlled so that the weight ratio of CzPA to 2PCAPA could be1:0.05 (=CzPA:2PCAPA).

Next, an electron transporting layer was formed by depositingtris(8-quinolinolato)aluminum(III) (abbreviation: Alq) to a thickness of30 nm over the light-emitting layer by a deposition method usingresistance heating. That is, unlike the light-emitting element 1, theelectron transporting layer was formed over the light-emitting layerwithout providing the layer for controlling the movement of carrierstherebetween. Then, an electron injection layer was formed by depositinglithium fluoride (LiF) to a thickness of 1 nm over the electrontransporting layer. Finally, a second electrode was formed by depositingaluminum to a thickness of 200 nm by a deposition method usingresistance heating. Consequently, the reference light-emitting element 2was formed.

The reference light-emitting element 2 obtained through theabove-described process was put into a glove box containing a nitrogenatmosphere so that the light-emitting element was sealed fromatmospheric air. Then, the operating characteristics of the referencelight-emitting element 2 were measured. Note that the measurement wasconducted at room temperature (atmosphere kept at 25° C.). The emissioncolor of the reference light-emitting element 2 was located at the CIEchromaticity coordinates of (x=0.29, y=0.62) at the luminance of 5000cd/m²; the current efficiency of the reference light-emitting element 2was 13 cd/A; and it exhibited green emission which derives from 2PCAPAsimilarly to the light-emitting element 1.

However, when a continuous lighting test was conducted in which thereference light-emitting element 2 was continuously lit by constantcurrent driving with the initial luminance set at 5000 cd/m², luminancehas decreased to 73% of the initial luminance after 740 hours. Thus, itwas found that the reference light-emitting element 2 has a shorterlifetime than the light-emitting element 1. Therefore, it was provedthat a long-lifetime light-emitting element can be obtained by applyingthe invention.

Embodiment 2

This embodiment will specifically describe fabrication examples of thelight-emitting element of the invention and the characteristics of thefabricated light-emitting element, with reference to the stackedstructure in FIG. 16. In addition, the characteristics will bespecifically described with reference to graphs showing the measurementresults. Structural formulae of organic compounds used in Embodiment 2are shown below. Note that the organic compounds that are alreadydescribed in Embodiment 1 are omitted.

(Fabrication of Light-Emitting Element 3)

First, ITO containing silicon oxide was deposited over a glass substrate2201 by a sputtering method, whereby a first electrode 2202 was formed.Note that the thickness of the first electrode 2202 was 110 nm and theelectrode area was 2 mm×2 mm. Next, the substrate having the firstelectrode 2202 was fixed to a substrate holder provided in a vacuumdeposition apparatus in such a way that the surface of the firstelectrode 2202 faced downward, and then the pressure was reduced toabout 10⁻⁴ Pa.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)and molybdenum(VI) oxide were co-deposited on the first electrode 2202,whereby a layer 2211 containing a composite material was formed. Thedeposition rate was controlled so that the thickness of the layer 2211could be 50 nm and the weight ratio of NPB to molybdenum(VI) oxide couldbe 4:1 (=NPB:molybdenum oxide). Then, a hole transporting layer 2212 wasformed by depositing 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB) to a thickness of 10 nm by a deposition method usingresistance heating.

Next, a light-emitting layer 2213 was formed over the hole transportinglayer 2212. The light-emitting layer 2213 was formed by co-depositing9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) to a thickness of 30 nm. Here, the depositionrate was controlled so that the weight ratio of CzPA to 2PCAPA could be1:0.05 (=CzPA:2PCAPA).

Further, a layer 2214 for controlling the movement of carriers wasformed by co-depositing tris(8-quinolinolato)aluminum(III)(abbreviation: Alq) and N,N′-diphenylquinacridon (abbreviation: DPQd) toa thickness of 10 nm over the light-emitting layer 2213. Here, thedeposition rate was controlled so that the weight ratio of Alq to DPQdcould be 1:0.005 (=Alq:DPQd).

After that, an electron transporting layer 2215 was formed by depositingbathophenanthroline (abbreviation: BPhen) to a thickness of 30 nm overthe layer 2214 for controlling the movement of carriers by a depositionmethod using resistance heating. Then, an electron injection layer 2216was formed by depositing lithium fluoride (LiF) to a thickness of 1 nmover the electron transporting layer 2215. Finally, a second electrode2204 was formed by depositing aluminum to a thickness of 200 nm by adeposition method using resistance heating. Consequently, alight-emitting element 3 was formed.

The light-emitting element 3 of the invention obtained through theabove-described process was put into a glove box containing a nitrogenatmosphere so that the light-emitting element was sealed fromatmospheric air. Then, the operating characteristics of thelight-emitting element 3 were measured. Note that the measurement wasconducted at room temperature (atmosphere kept at 25° C.). FIG. 22 showsthe current density vs. luminance characteristics of the light-emittingelement 3. FIG. 23 shows the voltage vs. luminance characteristics ofthe light-emitting element 3. FIG. 24 shows the luminance vs. currentefficiency characteristics of the light-emitting element 3. FIG. 25shows the emission spectrum of the light-emitting element 3 with acurrent supply of 1 mA.

FIG. 26 shows the result of a continuous lighting test in which thelight-emitting element 3 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² is100%). The emission color of the light-emitting element 3 was located atthe CIE chromaticity coordinates of (x=0.30, y=0.63) at the luminance of5000 cd/m², and green emission which derives from 2PCAPA was obtained.In addition, the current efficiency and driving voltage of thelight-emitting element 3 at the luminance of 5000 cd/m² were 13 cd/A and5.5 V, respectively. Further, when a continuous lighting test wasconducted in which the light-emitting element 3 was continuously lit byconstant current driving with the initial luminance set at 5000 cd/m²,83% of the initial luminance was maintained even after 570 hours. Thus,it was proved that the light-emitting element 3 has a long lifetime.

(Fabrication of Light-Emitting Element 4)

First, ITO containing silicon oxide was deposited over a glass substrate2201 by a sputtering method, whereby a first electrode 2202 was formed.Note that the thickness of the first electrode 2202 was 110 nm and theelectrode area was 2 mm×2 mm. Next, the substrate having the firstelectrode 2202 was fixed to a substrate holder provided in a vacuumdeposition apparatus in such a way that the surface of the firstelectrode 2202 faced downward, and then the pressure was reduced toabout 10⁻⁴ Pa.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)and molybdenum(VI) oxide were co-deposited on the first electrode 2202,whereby a layer 2211 containing a composite material was formed. Thedeposition rate was controlled so that the thickness of the layer 2211could be 50 nm and the weight ratio of NPB to molybdenum(VI) oxide couldbe 4:1 (=NPB:molybdenum oxide). Then, a hole transporting layer 2212 wasformed by depositing 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB) to a thickness of 10 nm by a deposition method usingresistance heating.

Next, a light-emitting layer 2213 was formed over the hole transportinglayer 2212. The light-emitting layer 2213 was formed by co-depositing9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) to a thickness of 30 nm. Here, the depositionrate was controlled so that the weight ratio of CzPA to 2PCAPA could be1:0.05 (=CzPA:2PCAPA).

Further, a layer 2214 for controlling the movement of carriers wasformed by co-depositing tris(8-quinolinolato)aluminum(III)(abbreviation: Alq) and9,18-dihydro-9,18-dimethyl-benzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-2) to a thickness of 10 nm over the light-emittinglayer 2213. Here, the deposition rate was controlled so that the weightratio of Alq to DMNQd-2 could be 1:0.005 (=Alq:DMNQd-2).

After that, an electron transporting layer 2215 was formed by depositingbathophenanthroline (abbreviation: BPhen) to a thickness of 30 nm overthe layer 2214 for controlling the movement of carriers by a depositionmethod using resistance heating. Then, an electron injection layer 2216was formed by depositing lithium fluoride (LiF) to a thickness of 1 nmover the electron transporting layer 2215. Finally, a second electrode2204 was formed by depositing aluminum to a thickness of 200 nm by adeposition method using resistance heating. Consequently, alight-emitting element 4 was formed.

The light-emitting element 4 of the invention obtained through theabove-described process was put into a glove box containing a nitrogenatmosphere so that the light-emitting element was sealed fromatmospheric air. Then, the operating characteristics of thelight-emitting element 4 were measured. Note that the measurement wasconducted at room temperature (atmosphere kept at 25° C.). FIG. 27 showsthe current density vs. luminance characteristics of the light-emittingelement 4. FIG. 28 shows the voltage vs. luminance characteristics ofthe light-emitting element 4. FIG. 29 shows the luminance vs. currentefficiency characteristics of the light-emitting element 4. FIG. 30shows the emission spectrum of the light-emitting element 4 with acurrent supply of 1 mA.

FIG. 26 shows the result of a continuous lighting test in which thelight-emitting element 4 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² is100%). The emission color of the light-emitting element 4 was located atthe CIE chromaticity coordinates of (x=0.30, y=0.64) at the luminance of5000 cd/m², and green emission which derives from 2PCAPA was obtained.In addition, the current efficiency and driving voltage of thelight-emitting element 4 at the luminance of 5000 cd/m² were 17 cd/A and4.2 V, respectively. Further, when a continuous lighting test wasconducted in which the light-emitting element 4 was continuously lit byconstant current driving with the initial luminance set at 5000 cd/m²,87% of the initial luminance was maintained even after 570 hours. Thus,it was proved that the light-emitting element 4 has a long lifetime.

(Fabrication of Light-Emitting Element 5)

First, ITO containing silicon oxide was deposited over a glass substrate2201 by a sputtering method, whereby a first electrode 2202 was formed.Note that the thickness of the first electrode 2202 was 110 nm and theelectrode area was 2 mm×2 mm. Next, the substrate having the firstelectrode 2202 was fixed to a substrate holder provided in a vacuumdeposition apparatus in such a way that the surface of the firstelectrode 2202 faced downward, and then the pressure was reduced toabout 10⁻⁴ Pa.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)and molybdenum(VI) oxide were co-deposited on the first electrode 2202,whereby a layer 2211 containing a composite material was formed. Thedeposition rate was controlled so that the thickness of the layer 2211could be 50 nm and the weight ratio of NPB to molybdenum(VI) oxide couldbe 4:1 (=NPB:molybdenum oxide). Then, a hole transporting layer 2212 wasformed by depositing 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB) to a thickness of 10 nm by a deposition method usingresistance heating.

Next, a light-emitting layer 2213 was formed over the hole transportinglayer 2212. The light-emitting layer 2213 was formed by co-depositing9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) to a thickness of 30 nm. Here, the depositionrate was controlled so that the weight ratio of CzPA to 2PCAPA could be1:0.05 (=CzPA:2PCAPA).

Further, a layer 2214 for controlling the movement of carriers wasformed by co-depositing tris(8-quinolinolato)aluminum(III)(abbreviation: Alq) and Coumarin 6 to a thickness of 10 nm over thelight-emitting layer 2213. Here, the deposition rate was controlled sothat the weight ratio of Alq to Coumarin 6 could be 1:0.01 (=AlqCoumarin 6).

After that, an electron transporting layer 2215 was formed by depositingbathophenanthroline (abbreviation: BPhen) to a thickness of 30 nm overthe layer 2214 for controlling the movement of carriers by a depositionmethod using resistance heating. Then, an electron injection layer 2216was formed by depositing lithium fluoride (LiF) to a thickness of 1 nmover the electron transporting layer 2215. Finally, a second electrode2204 was formed by depositing aluminum to a thickness of 200 nm by adeposition method using resistance heating. Consequently, alight-emitting element 5 was formed.

The light-emitting element 5 of the invention obtained through theabove-described process was put into a glove box containing a nitrogenatmosphere so that the light-emitting element was sealed fromatmospheric air. Then, the operating characteristics of thelight-emitting element 5 were measured. Note that the measurement wasconducted at room temperature (atmosphere kept at 25° C.). FIG. 31 showsthe current density vs. luminance characteristics of the light-emittingelement 5. FIG. 32 shows the voltage vs. luminance characteristics ofthe light-emitting element 5. FIG. 33 shows the luminance vs. currentefficiency characteristics of the light-emitting element 5. FIG. 34shows the emission spectrum of the light-emitting element 5 with acurrent supply of 1 mA.

FIG. 26 shows the result of a continuous lighting test in which thelight-emitting element 5 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² is100%). The emission color of the light-emitting element 5 was located atthe CIE chromaticity coordinates of (x=0.30, y=0.63) at the luminance of5000 cd/m², and green emission which derives from 2PCAPA was obtained.In addition, the current efficiency and driving voltage of thelight-emitting element 5 at the luminance of 5000 cd/m² were 16 cd/A and4.5 V, respectively. Further, when a continuous lighting test wasconducted in which the light-emitting element 5 was continuously lit byconstant current driving with the initial luminance set at 5000 cd/m²,93% of the initial luminance was maintained even after 150 hours. Thus,it was proved that the light-emitting element 5 has a long lifetime.

(Fabrication of Reference Light-Emitting Element 6)

Next, for the sake of comparison, a reference light-emitting element 6was formed, which differs from the above-described light-emittingelements 3 to 5 only in that the layer 2214 for controlling the movementof carriers is formed without using an organic compound having anelectron trapping property (i.e., the layer 2214 was formed using onlyAlq). Thus, the reference light-emitting element 6 was formed in asimilar manner to the light-emitting elements 3 to 5 except that point.The fabrication method will be described below. First, ITO containingsilicon oxide was deposited over a glass substrate by a sputteringmethod, whereby a first electrode was formed. Note that the thickness ofthe first electrode was 110 nm and the electrode area was 2 mm×2 mm.

Next, the substrate having the first electrode was fixed to a substrateholder provided in a vacuum deposition apparatus in such a way that thesurface of the first electrode faced downward, and then the pressure wasreduced to about 10⁻⁴ Pa. Then,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-deposited on the first electrode, whereby alayer containing a composite material was formed. The deposition ratewas controlled so that the thickness of the layer containing a compositematerial could be 50 nm and the weight ratio of NPB to molybdenum(VI)oxide could be 4:1 (=NPB:molybdenum oxide).

Next, a hole transporting layer was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) to athickness of 10 nm by a deposition method using resistance heating.Then, a light-emitting layer was formed over the hole transportinglayer. The light-emitting layer was formed by co-depositing9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) to a thickness of 30 nm. Here, the depositionrate was controlled so that the weight ratio of CzPA to 2PCAPA could be1:0.05 (=CzPA:2PCAPA).

Next, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) was formedto a thickness of 10 nm over the light-emitting layer. That is, a layermade of only Alq was formed unlike the light-emitting elements 3 to 5.After that, an electron transporting layer was formed by depositingbathophenanthroline (abbreviation: BPhen) to a thickness of 30 nm overthe layer made of only Alq by a deposition method using resistanceheating. Further, an electron injection layer was formed by depositinglithium fluoride (LiF) to a thickness of 1 nm over the electrontransporting layer.

Finally, a second electrode was formed by depositing aluminum to athickness of 200 nm by a deposition method using resistance heating.Consequently, the reference light-emitting element 6 was formed. Thereference light-emitting element 6 obtained through the above-describedprocess was put into a glove box containing a nitrogen atmosphere sothat the light-emitting element was sealed from atmospheric air. Then,the operating characteristics of the reference light-emitting element 6were measured. Note that the measurement was conducted at roomtemperature (atmosphere kept at 25° C.).

The emission color of the reference light-emitting element 6 was locatedat the CIE chromaticity coordinates of (x=0.28, y=0.64) at the luminanceof 5000 cd/m²; the current efficiency of the reference light-emittingelement 6 was 18 cd/A; and it exhibited green emission which derivesfrom 2PCAPA similarly to the light-emitting elements 3 to 5. However,when a continuous lighting test was conducted in which the referencelight-emitting element 6 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m², luminance hasdecreased to 75% of the initial luminance after 260 hours. Thus, it wasfound that the reference light-emitting element 6 has a shorter lifetimethan the light-emitting elements 3 to 5. Therefore, it was proved that along-lifetime light-emitting element can be obtained by applying theinvention.

Embodiment 3

In this embodiment, reduction reaction characteristics oftris(8-quinolinolato)aluminum(III) (abbreviation: Alq),N,N′-diphenylquinacridon (abbreviation: DPQd), and Coumarin 6, which areused for the layer for controlling the movement of carriers in thelight-emitting elements 1, 3, and 5 fabricated in Embodiments 1 and 2,were observed by cyclic voltammetry (CV) measurement. Further, the LUMOlevels of Alq, DPQd, and Coumarin 6 were determined from the measurementresults. Note that an electrochemical analyzer (ALS model 600A or 600C,product of BAS Inc.) was used for the measurement.

As for a solution used in the CV measurement, dehydrateddimethylformamide (DMF, product of Sigma-Aldrich Inc., 99.8%, catalogNo. 22705-6) was used for a solvent, and Tetra-n-butylammoniumperchlorate (n-Bu₄NClO₄, product of Tokyo Chemical Industry Co., Ltd.,catalog No. T0836), which is a supporting electrolyte, was dissolved inthe solvent such that the concentration of tetra-n-butylammoniumperchlorate was 100 mmol/L. Further, the object to be measured was alsodissolved in the solvent such that the concentration thereof was 1mmol/L. In addition, a platinum electrode (a PTE platinum electrode,product of BAS Inc.) was used as a working electrode; a platinumelectrode (a VC-3 Pt counter electrode (5 cm), product of BAS Inc.) wasused as an auxiliary electrode; and an Ag/Ag⁺ electrode (an RE5nonaqueous solvent reference electrode, product of BAS Inc.) was used asa reference electrode. Note that the measurement was conducted at roomtemperature (20 to 25° C.).

[Calculation of the Potential Energy of the Reference Electrode withRespect to the Vacuum Level]

First, potential energy (eV) of the reference electrode (Ag/Ag⁺electrode) used in Embodiment 3 with respect to the vacuum level wascalculated. That is, the Fermi level of the Ag/Ag⁺ electrode wascalculated. It is known that the oxidation-reduction potential offerrocene in methanol is +0.610 V [vs. SHE] with respect to a standardhydrogen electrode (Reference: Christian R. Goldsmith et al., J. Am.Chem. Soc., Vol. 124, No. 1, pp. 83-96, 2002). On the other hand, whenthe oxidation-reduction potential of ferrocene in methanol wascalculated using the reference electrode used in Embodiment 3, theresult was +0.20 V [vs. Ag/Ag⁺].

Therefore, it was found that the potential energy of the referenceelectrode used in Embodiment 3 was lower than that of the standardhydrogen electrode by 0.41 [eV]. Here, it is also known that thepotential energy of the standard hydrogen electrode with respect to thevacuum level is −4.44 eV (Reference: Toshihiro Ohnishi and TamamiKoyama, High molecular EL material, Kyoritsu shuppan, pp. 64-67).Accordingly, the potential energy of the reference electrode used inEmbodiment 3 with respect to the vacuum level could be determined to be−4.44−0.41=−4.85 [eV].

MEASUREMENT EXAMPLE 1 Alq

In Measurement Example 1, the reduction reaction characteristics of Alqwere observed by cyclic voltammetry (CV) measurement. The scan rate wasset at 0.1 V/sec. FIG. 35 shows the measurement result. Note that themeasurement of the reduction reaction characteristics was conducted bythe steps of: scanning the potential of the working electrode withrespect to the reference electrode in ranges of (1) −0.69 V to −2.40 V,and then (2) −2.40 V to −0.69 V.

As shown in FIG. 35, it can be seen that a reduction peak potentialE_(pc) is −2.20 V and an oxidation peak potential E_(pa) is −2.12 V.Therefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be determined to be −2.16 V. This shows that Alqcan be reduced by an electrical energy of −2.16 V [vs. Ag/Ag⁺], and thisenergy corresponds to the LUMO level. Here, the potential energy of thereference electrode used in Embodiment 3 with respect to the vacuumlevel is −4.85 [eV] as described above. Therefore, the LUMO level of Alqcan be determined to be −4.85−(−2.16)=−2.69 [eV].

MEASUREMENT EXAMPLE 2 DPQd

In Measurement Example 2, the reduction reaction characteristics of DPQdwere observed by cyclic voltammetry (CV) measurement. The scan rate wasset at 0.1 V/sec. FIG. 36 shows the measurement result. Note that themeasurement of the reduction reaction characteristics was conducted bythe steps of: scanning the potential of the working electrode withrespect to the reference electrode in ranges of (1) −0.40 V to −2.10 Vand then (2) −2.10 V to −0.40 V. In addition, since DPQd which has lowsolubility could not be completely dissolved in a solvent even when thesolution was adjusted to contain DPQd at a concentration of 1 mmol/L,the measurement was conducted by taking a supernatant liquid while anundissolved portion is precipitated at the bottom.

As shown in FIG. 36, it can be seen that a reduction peak potentialE_(pc) is −1.69 V and an oxidation peak potential E_(pa) is −1.63 V.Therefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be determined to be −1.66 V. This shows that DPQdcan be reduced by an electrical energy of −1.16 V [vs. Ag/Ag⁺], and thisenergy corresponds to the LUMO level. Here, the potential energy of thereference electrode used in Embodiment 3 with respect to the vacuumlevel is −4.85 [eV] as described above. Therefore, the LUMO level ofDPQd can be determined to be −4.85−(−1.16)=−3.19 [eV].

Note that when the LUMO levels of Alq and DPQd which were calculated inthe above-described manner are compared, it can be found that the LUMOlevel of DPQd is lower than that of Alq by as much as 0.50 [eV]. Thismeans that DPQd can function as electron traps when added into Alq.Therefore, for the light-emitting element of the invention, it is quiteadvantageous to use the element structure shown in Embodiments 1 and 2in which DPQd is used as the second organic compound of the second layerand Alq is used as the first organic compound thereof.

Note that9,18-dihydro-9,18-dimethyl-benzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-2) used for the light-emitting element 4 hasextremely low solubility, and thus it could not be measured by CV.DMNQd-2 is a quinacridone derivative like DPQd. Therefore, it has acarbonyl group in its molecular skeleton and thus has a strong electrontrapping property. Therefore, DMNQd-2 can be considered to have similarphysical properties to DPQd.

MEASUREMENT EXAMPLE 3 Coumarin 6

In Measurement Example 3, the reduction reaction characteristics ofCoumarin 6 were observed by cyclic voltammetry (CV) measurement. Thescan rate was set at 0.1 V/sec. FIG. 37 shows the measurement result.Note that the measurement of the reduction reaction characteristics wasconducted by the steps of: scanning the potential of the workingelectrode with respect to the reference electrode in ranges of (1) −0.31V to −2.00 V, and then (2) −2.00 V to −0.31 V.

As shown in FIG. 37 it can be seen that a reduction peak potentialE_(pc) is −1.85 V and an oxidation peak potential E_(pa) is −1.77 V.Therefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be determined to be −1.81 V. This shows thatCoumarin 6 can be reduced by an electrical energy of −1.81 V [vs.Ag/Ag⁺], and this energy corresponds to the LUMO level. Here, thepotential energy of the reference electrode used in Embodiment 3 withrespect to the vacuum level is −4.85 [eV] as described above. Therefore,the LUMO level of Coumarin 6 can be determined to be −4.85−(−1.81)=−3.04[eV].

Note that when the LUMO levels of Alq and Coumarin 6 which werecalculated in the above-described manner are compared, it can be foundthat the LUMO level of Coumarin 6 is lower than that of Alq by as muchas 0.35 [eV]. This means that Coumarin 6 can function as electron trapswhen added into Alq. Therefore, for the light-emitting element of theinvention, it is quite advantageous to use the element structure shownin Embodiment 2 in which Coumarin 6 is used as the second organiccompound of the layer for controlling the movement of carriers and Alqis used as the first organic compound thereof.

Embodiment 4

This embodiment will specifically describe a fabrication example of thelight-emitting element of the invention and the characteristics of thefabricated light-emitting element, with reference to the stackedstructure in FIG. 16. In addition, the characteristics will bespecifically described with reference to graphs showing the measurementresults.

(Fabrication of Light-Emitting Element 7)

First, ITO containing silicon oxide was deposited over a glass substrate2201 by a sputtering method, whereby a first electrode 2202 was formed.Note that the thickness of the first electrode 2202 was 110 nm and theelectrode area was 2 mm×2 mm.

Next, the substrate having the first electrode 2202 was fixed to asubstrate holder provided in a vacuum deposition apparatus in such a waythat the surface of the first electrode 2202 faced downward, and thenthe pressure was reduced to about 10⁻⁴ Pa. Then,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-deposited on the first electrode 2202,whereby a layer 2211 containing a composite material was formed. Thedeposition rate was controlled so that the thickness of the layer 2211could be 50 nm and the weight ratio of NPB to molybdenum(VI) oxide couldbe 4:1 (=NPB:molybdenum oxide). Note that the co-deposition method is adeposition method in which deposition is performed using a plurality ofevaporation sources at the same time in one treatment chamber.

Next, a hole transporting layer 2212 was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) to athickness of 10 nm by a deposition method using resistance heating.Then, a light-emitting layer 2213 was formed over the hole transportinglayer 2212. The light-emitting layer 2213 was formed by co-depositing9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) to a thickness of 30 nm. Here, the depositionrate was controlled so that the weight ratio of CzPA to 2PCAPA could be1:0.05 (=CzPA:2PCAPA).

Further, a layer 2214 for controlling the movement of carriers wasformed by co-depositing tris(8-quinolinolato)aluminum(III)(abbreviation: Alq) and N,N′-diphenylquinacridon (abbreviation: DPQd) toa thickness of 10 nm over the light-emitting layer 2213. Here, thedeposition rate was controlled so that the weight ratio of Alq to DPQdcould be 1:0.002 (=Alq:DPQd).

After that, an electron transporting layer 2215 was formed by depositingtris(8-quinolinolato)aluminum(III) (abbreviation: Alq) to a thickness of30 nm over the layer 2214 for controlling the movement of carriers by adeposition method using resistance heating. Then, an electron injectionlayer 2216 was formed by depositing lithium fluoride (LiF) to athickness of 1 nm over the electron transporting layer 2215.

Finally, a second electrode 2204 was formed by depositing aluminum to athickness of 200 nm by a deposition method using resistance heating.Consequently, a light-emitting element 7 was formed. The light-emittingelement 7 of the invention obtained through the above-described processwas put into a glove box containing a nitrogen atmosphere so that thelight-emitting element was sealed from atmospheric air. Then, theoperating characteristics of the light-emitting element 7 were measured.Note that the measurement was conducted at room temperature (atmospherekept at 25-C). FIG. 38 shows the current density vs. luminancecharacteristics of the light-emitting element 7. FIG. 39 shows thevoltage vs. luminance characteristics of the light-emitting element 7.FIG. 40 shows the luminance vs. current efficiency characteristics ofthe light-emitting element 7. FIG. 41 shows the emission spectrum of thelight-emitting element 7 with a current supply of 1 mA.

FIG. 42 shows the result of a continuous lighting test in which thelight-emitting element 7 was continuously lit by constant currentdriving at 80° C. with the initial luminance set at 1000 cd/m² (thevertical axis indicates the relative luminance on the assumption that1000 cd/m² is 100%). The emission color of the light-emitting element 7was located at the CIE chromaticity coordinates of (x=0.28, y=0.65) atthe luminance of 1040 cd/m², and green emission which derives from2PCAPA was obtained. In addition, the current efficiency, drivingvoltage, and power efficiency of the light-emitting element 7 at theluminance of 1040 cd/m² were 16 cd/A, 5.4 V, and 9.11 m/W, respectively.

Further, when a continuous lighting test was conducted in which thelight-emitting element 7 was continuously lit by constant currentdriving at 80° C. with the initial luminance set at 1000 cd/m², 96% ofthe initial luminance was maintained even after 1000 hours, and yet 94%of the initial luminance was maintained even after 1700 hours. Thus, alight-emitting element having a very long lifetime even in thehigh-temperature environment of 80° C. could be obtained. Therefore, itwas proved that a long-lifetime light-emitting element can be obtainedby applying the invention.

Embodiment 5

This embodiment will specifically describe a fabrication example of thelight-emitting element of the invention which differs from that shown inEmbodiment 4, and the characteristics of the fabricated light-emittingelement, with reference to the stacked structure in FIG. 16. Inaddition, the characteristics will be specifically described withreference to graphs showing the measurement results.

(Fabrication of Light-Emitting Element 8)

First, ITO containing silicon oxide was deposited over a glass substrate2201 by a sputtering method, whereby a first electrode 2202 was formed.Note that the thickness of the first electrode 2202 was 110 nm and theelectrode area was 2 mm×2 mm.

Next, the substrate having the first electrode 2202 was fixed to asubstrate holder provided in a vacuum deposition apparatus in such a waythat the surface of the first electrode 2202 faced downward, and thenthe pressure was reduced to about 10⁻⁴ Pa. Then,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-deposited on the first electrode 2202,whereby a layer 2211 containing a composite material was formed. Thedeposition rate was controlled so that the thickness of the layer 2211could be 50 nm and the weight ratio of NPB to molybdenum(VI) oxide couldbe 4:1 (=NPB:molybdenum oxide).

Next, a hole transporting layer 2212 was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) to athickness of 10 nm by a deposition method using resistance heating.Further, a light-emitting layer 2213 was formed over the holetransporting layer 2212. The light-emitting layer 2213 was formed byco-depositing 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) to a thickness of 30 nm. Here, the depositionrate was controlled so that the weight ratio of CzPA to 2PCAPA could be1:0.05 (=CzPA:2PCAPA).

Further, a layer 2214 for controlling the movement of carriers wasformed by co-depositing tris(8-quinolinolato)aluminum(III)(abbreviation: Alq) and N,N′-diphenylquinacridon (abbreviation: DPQd) toa thickness of 10 nm over the light-emitting layer 2213. Here, thedeposition rate was controlled so that the weight ratio of Alq to DPQdcould be 1:0.002 (=Alq:DPQd).

After that, an electron transporting layer 2215 was formed by depositingbathophenanthroline (abbreviation: BPhen) to a thickness of 30 nm overthe layer 2214 for controlling the movement of carriers by a depositionmethod using resistance heating. Then, an electron injection layer 2216was formed by depositing lithium fluoride (LiF) to a thickness of 1 nmover the electron transporting layer 2215. Finally, a second electrode2204 was formed by depositing aluminum to a thickness of 200 nm by adeposition method using resistance heating. Consequently, alight-emitting element 8 was formed.

The light-emitting element 8 of the invention obtained through theabove-described process was put into a glove box containing a nitrogenatmosphere so that the light-emitting element was sealed fromatmospheric air. Then, the operating characteristics of thelight-emitting element 8 were measured. Note that the measurement wasconducted at room temperature (atmosphere kept at 25° C.). FIG. 43 showsthe current density and luminance of the light-emitting element 8. FIG.44 shows the voltage vs. luminance characteristics of the light-emittingelement 8. FIG. 45 shows the luminance vs. current efficiencycharacteristics of the light-emitting element 8. FIG. 46 shows theemission spectrum of the light-emitting element 8 with a current supplyof 1 mA.

In addition, FIG. 47 shows the result of a continuous lighting test inwhich the light-emitting element 8 was continuously lit by constantcurrent driving with the initial luminance set at 1000 cd/m² (thevertical axis indicates the relative luminance on the assumption that1000 cd/m² is 100%). The emission color of the light-emitting element 8was located at the CIE chromaticity coordinates of (x=0.27, y=0.65) atthe luminance of 1440 cd/m², and green emission which derives from2PCAPA was obtained. In addition, the current efficiency, drivingvoltage, and power efficiency of the light-emitting element 8 at theluminance of 1440 cd/m² were 17 cd/A, 3.8 V, and 141 m/W, respectively.

Further, when a continuous lighting test was conducted in which thelight-emitting element 8 was continuously lit by constant currentdriving with the initial luminance set at 1000 cd/m², 99% of the initialluminance was maintained even after 3300 hours. Thus, the light-emittingelement 8 was found to have a long lifetime. Therefore, it was provedthat a long-lifetime light-emitting element can be obtained by applyingthe invention.

Embodiment 6

This embodiment will specifically describe fabrication examples of thelight-emitting element of the invention which differs from those shownin Embodiments 4 and 5, and the characteristics of the fabricatedlight-emitting element, with reference to the stacked structure in FIG.48. In addition, the characteristics will be specifically described withreference to graphs showing the measurement results. Structural formulaeof organic compounds used in Embodiment 6 are shown below. Note that theorganic compounds that are already described in the precedingembodiments are omitted.

(Fabrication of Light-Emitting Element 9)

First, ITO containing silicon oxide was deposited over a glass substrate2401 by a sputtering method, whereby a first electrode 2402 was formed.Note that the thickness of the first electrode 2402 was 110 nm and theelectrode area was 2 mm×2 mm. Next, the substrate having the firstelectrode 2402 was fixed to a substrate holder provided in a vacuumdeposition apparatus in such a way that the surface of the firstelectrode 2402 faced downward, and then the pressure was reduced toabout 10⁻⁴ Pa.

Then, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)and molybdenum(VI) oxide were co-deposited on the first electrode 2402,whereby a layer 2411 containing a composite material was formed. Thedeposition rate was controlled so that the thickness of the layer 2411could be 50 nm and the weight ratio of NPB to molybdenum(VI) oxide couldbe 4:1 (=NPB:molybdenum oxide).

Next, a hole transporting layer 2412 was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) to athickness of 10 nm by a deposition method using resistance heating.Further, a layer 2413 for controlling the movement of carriers wasformed by co-depositing2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD) to a thickness of 10 nm over the hole transportinglayer 2412. Here, the deposition rate was controlled so that the weightratio of BPAPQ to DNTPD could be 1:0.1 (=BPAPQ:DNTPD).

Then, a light-emitting layer 2414 was formed over the layer 2413 forcontrolling the movement of carriers. The light-emitting layer 2414 wasformed by co-depositing2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)) to a thickness of 20 nm. Here, thedeposition rate was controlled so that the weight ratio of BPAPQ toIr(Fdpq)₂(acac) could be 1:0.07 (=BPAPQ Ir(Fdpq)₂(acac)).

After that, an electron transporting layer 2415 was formed by depositingtris(8-quinolinolato)aluminum(III) (abbreviation: Alq) to a thickness of10 nm over the light-emitting layer 2414. Then, an electron injectionlayer 2416 was formed by co-depositingtris(8-quinolinolato)aluminum(III) (abbreviation: Alq) and lithium (Li)to a thickness of 50 nm over the electron transporting layer 2415. Here,the deposition rate was controlled so that the weight ratio of Alq to Licould be 1:0.01 (=Alq:Li).

Finally, a second electrode 2404 was formed by depositing aluminum to athickness of 200 nm by a deposition method using resistance heating.Consequently, a light-emitting element 9 was formed. The light-emittingelement 9 of the invention obtained through the above-described processwas put into a glove box containing a nitrogen atmosphere so that thelight-emitting element was sealed from atmospheric air. Then, theoperating characteristics of the light-emitting element 9 were measured.Note that the measurement was conducted at room temperature (atmospherekept at 25° C.).

FIG. 49 shows the current density vs. luminance characteristics of thelight-emitting element 9. FIG. 50 shows the voltage vs. luminancecharacteristics of the light-emitting element 9. FIG. 51 shows theluminance vs. current efficiency characteristics of the light-emittingelement 9. FIG. 52 shows the emission spectrum of the light-emittingelement 9 with a current supply of 1 mA. In addition, FIG. 53 shows theresult of a continuous lighting test in which the light-emitting element9 was continuously lit by constant current driving with the initialluminance set at 1000 cd/m² (the vertical axis indicates the relativeluminance on the assumption that 1000 cd/m² is 100%).

The emission color of the light-emitting element 9 was located at theCIE chromaticity coordinates of (x=0.70, y=0.30) at the luminance of1060 cd/m², and red emission which derives from Ir(Fdpq)₂(acac) wasobtained. In addition, the current efficiency, driving voltage, andpower efficiency of the light-emitting element 9 at the luminance of1060 cd/m² were 4.7 cd/A, 6.2 V, and 2.41 m/W, respectively. Further,when a continuous lighting test was conducted in which thelight-emitting element 9 was continuously lit by constant currentdriving with the initial luminance set at 1000 cd/m², 74% of the initialluminance was maintained even after 2400 hours. Thus, it was proved thatthe light-emitting element 9 has a long lifetime.

(Fabrication of Light-Emitting Element 10)

A light-emitting element 10 was formed by using2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and4,4′4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1-TNATA) for the layer 2413 for controlling the movement of carriers ofthe light-emitting element 9.

That is, the layer 2413 for controlling the movement of carriers wasformed by co-depositing2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and4,4′4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1-TNATA) to a thickness of 10 nm over the hole transporting layer 2412.Here, the deposition rate was controlled so that the weight ratio ofBPAPQ to 1-TNATA could be 1:0.1 (=BPAPQ: 1-TNATA). Note that layersother than the layer 2413 for controlling the movement of carriers wereformed in a similar manner to those of the light-emitting element 9.

The light-emitting element 10 of the invention obtained through theabove-described process was put into a glove box containing a nitrogenatmosphere so that the light-emitting element was sealed fromatmospheric air. Then, the operating characteristics of thelight-emitting element 10 were measured. Note that the measurement wasconducted at room temperature (atmosphere kept at 25° C.). FIG. 54 showsthe current density vs. luminance characteristics of the light-emittingelement 10. FIG. 55 shows the voltage vs. luminance characteristics ofthe light-emitting element 10. FIG. 56 shows the luminance vs. currentefficiency characteristics of the light-emitting element 10. FIG. 57shows the emission spectrum of the light-emitting element 10 with acurrent supply of 1 mA. In addition, FIG. 58 shows the result of acontinuous lighting test in which the light-emitting element 10 wascontinuously lit by constant current driving with the initial luminanceset at 1000 cd/m² (the vertical axis indicates the relative luminance onthe assumption that 1000 cd/m² is 100%).

The emission color of the light-emitting element 10 was located at theCIE chromaticity Coordinates of (x=0.70, y=0.30) at the luminance of 960cd/m², and red emission which derives from Ir(Fdpq)₂(acac) was obtained.In addition, the current efficiency, driving voltage, and powerefficiency of the light-emitting element 10 at the luminance of 960cd/m² were 5.1 cd/A, 7.0 V, and 2.31 m/W, respectively. Further, when acontinuous lighting test was conducted in which the light-emittingelement 10 was continuously lit by constant current driving with theinitial luminance set at 1000 cd/m², 84% of the initial luminance wasmaintained even after 1100 hours. Thus, it was proved that thelight-emitting element 10 has a long lifetime.

(Fabrication of Reference Light-Emitting Element 11)

Next, for the sake of comparison, a reference light-emitting element 11was formed, which does not include the layer 2413 for controlling themovement of carriers unlike the above-described light-emitting elements9 and 10. First, ITO containing silicon oxide was deposited over a glasssubstrate by a sputtering method, whereby a first electrode was formed.Note that the thickness of the first electrode was 110 nm and theelectrode area was 2 mm×2 mm.

Next, the substrate having the first electrode was fixed to a substrateholder provided in a vacuum deposition apparatus in such a way that thesurface of the first electrode faced downward, and then the pressure wasreduced to about 10⁻⁴ Pa. Then,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-deposited on the first electrode, whereby alayer containing a composite material was formed. The deposition ratewas controlled so that the thickness of the layer containing a compositematerial could be 50 nm and the weight ratio of NPB to molybdenum(VI)oxide could be 4:1 (=NPB:molybdenum oxide). After that, a holetransporting layer was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) to athickness of 10 nm by a deposition method using resistance heating.

Next, a light-emitting layer was formed over the hole transportinglayer. The light-emitting layer was formed by co-depositing2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)) to a thickness of 30 nm. Here, thedeposition rate was controlled so that the weight ratio of BPAPQ toIr(Fdpq)₂(acac) could be 1:0.07 (=BPAPQ:Ir(Fdpq)₂(acac)). That is,unlike the light-emitting elements 9 and 10, the light-emitting layerwas formed over the hole transporting layer without providing the layerfor controlling the movement of carriers therebetween.

After that, an electron transporting layer was formed by depositingtris(8-quinolinolato)aluminum(III) (abbreviation: Alq) to a thickness of10 nm over the light-emitting layer by a deposition method usingresistance heating. Then, an electron injection layer was formed byco-depositing tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) andlithium (Li) to a thickness of 50 nm over the electron transportinglayer. Here, the deposition rate was controlled so that the weight ratioof Alq to Li could be 1:0.01 (=Alq:Li).

Finally, a second electrode was formed by depositing aluminum to athickness of 200 nm by a deposition method using resistance heating.Consequently, the reference light-emitting element 11 was formed. Thereference light-emitting element 11 obtained through the above-describedprocess was put into a glove box containing a nitrogen atmosphere sothat the light-emitting element was sealed from atmospheric air. Then,the operating characteristics of the light-emitting element 11 weremeasured. Note that the measurement was conducted at room temperature(atmosphere kept at 25° C.).

FIG. 59 shows the current density vs. luminance characteristics of thelight-emitting element 11. FIG. 60 shows the voltage vs. luminancecharacteristics of the light-emitting element 11. FIG. 61 shows theluminance vs. current efficiency characteristics of the light-emittingelement 11. FIG. 62 shows the emission spectrum of the light-emittingelement 11 with a current supply of 1 mA. In addition, FIG. 63 shows theresult of a continuous lighting test in which the light-emitting element11 was continuously lit by constant current driving with the initialluminance set at 1000 cd/m² (the vertical axis indicates the relativeluminance on the assumption that 1000 cd/m² is 100%).

The emission color of the reference light-emitting element 11 waslocated at the CIE chromaticity coordinates of (x=0.65, y=0.34) at theluminance of 1140 cd/m², and red emission which derives fromIr(Fdpq)₂(acac) was obtained. However, as is apparent from FIG. 62,emission of Alq which is in contact with the light-emitting layer isalso seen, which means the color purity of the reference light-emittingelement 11 is lower than those of the light-emitting elements 9 and 10.That is, it can be said that the reference light-emitting element 11 hasa worse carrier balance than the light-emitting elements 9 and 10.

In addition, the current efficiency, driving voltage, and powerefficiency of the reference light-emitting element 11 at the luminanceof 1140 cd/m² were 2.9 cd/A, 4.8 V, and 1.91 m/W, respectively. Further,when a continuous lighting test was conducted in which the referencelight-emitting element 11 was continuously lit by constant currentdriving with the initial luminance set at 1000 cd/m², luminance hasdecreased to 73% of the initial luminance after 1300 hours. Thus, it wasfound that the reference light-emitting element 11 has a shorterlifetime than the light-emitting elements 9 and 10. Therefore, it wasproved that a long-lifetime light-emitting element can be obtained byapplying the invention.

Embodiment 7

In this embodiment, oxidation reaction characteristics of2,3-bis{4-[N-(4-biphenylyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD), which are used for the layer for controlling themovement of carriers in the light-emitting element 9 formed inEmbodiment 6, were observed by cyclic voltammetry (CV) measurement.Further, the HOMO levels of BPAPQ and DNTPD were determined from themeasurement results. Note that an electrochemical analyzer (ALS model600A or 600C, product of BAS Inc.) was used for the measurement.

The CV measurement was conducted in a similar manner to Embodiment 4. Asfor a solution used in the CV measurement, dehydrated dimethylformamide(DMF, product of Sigma-Aldrich Co., 99.8%, catalog No. 22705-6) was usedfor a solvent, and Tetra-n-butylammonium perchlorate (n-Bu₄NClO₄,product of Tokyo Chemical Industry Co., Ltd., catalog No. T0836), whichis a supporting electrolyte, was dissolved in the solvent such that theconcentration of tetra-n-butylammonium perchlorate was 100 mmol/L.Further, the object to be measured was also dissolved in the solventsuch that the concentration thereof was 1 mmol/L. In addition, aplatinum electrode (a PTE platinum electrode, product of BAS Inc.) wasused as a working electrode; a platinum electrode (a VC-3 Pt counterelectrode (5 cm), product of BAS Inc.) was used as an auxiliaryelectrode; and an Ag/Ag⁺ electrode (an RE5 nonaqueous solvent referenceelectrode, product of BAS Inc.) was used as a reference electrode. Notethat the measurement was conducted at room temperature (20 to 25° C.).

[Calculation of the Potential Energy of the Reference Electrode withRespect to the Vacuum Level]

First, potential energy (eV) of the reference electrode (Ag/Ag⁺electrode) used in Embodiment 7 with respect to the vacuum level wascalculated by the method described in Embodiment 3. As a result, thepotential energy of the reference electrode used in Embodiment 7 withrespect to the vacuum level could be determined to be −4.44−0.41=−4.85[eV]

MEASUREMENT EXAMPLE 4 BPAPQ

In Measurement Example 4, the oxidation reaction characteristics ofBPAPQ were observed by cyclic voltammetry (CV) measurement. The scanrate was set at 0.1 V/sec. FIG. 64 shows the measurement result. Notethat the measurement of the oxidation reaction characteristics wasconducted by the steps of: scanning the potential of the workingelectrode with respect to the reference electrode in ranges of (1) −0.16V to 1.00 V, and then (2) 1.00 V to −0.16 V.

As shown in FIG. 64, it can be seen that an oxidation peak potentialE_(pa) is 0.78 V and a reduction peak potential E_(pc) is 0.60 V.Therefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be determined to be 0.69 V. This shows that BPAPQcan be oxidized by an electrical energy of 0.69 V [vs. Ag/Ag⁺], and thisenergy corresponds to the HOMO level. Here, the potential energy of thereference electrode used in Embodiment 7 with respect to the vacuumlevel is −4.85 [eV] as described above. Therefore, the HOMO level ofBPAPQ can be determined to be −4.85−0.69=−5.54 [eV].

MEASUREMENT EXAMPLE 5 DNTPD

In Measurement Example 5, the oxidation reaction characteristics ofDNTPD were observed by cyclic voltammetry (CV) measurement. The scanrate was set at 0.1 V/sec. FIG. 65 shows the measurement result. Notethat the measurement of the oxidation reaction characteristics wasconducted by the steps of: scanning the potential of the workingelectrode with respect to the reference electrode in ranges of (1) −0.05V to 1.20 V, and then (2) 1.20 V to −0.05 V.

As shown in FIG. 65, it can be seen that an oxidation peak potentialE_(pa) is 0.16 V and a reduction peak potential E_(pc) is 0.26 V.Therefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be determined to be 0.21 V. This shows that DPQdcan be oxidized by an electrical energy of 0.21 V [vs. Ag/Ag⁺], and thisenergy corresponds to the HOMO level. Here, the potential energy of thereference electrode used in Embodiment 7 with respect to the vacuumlevel is −4.85 [eV] as described above. Therefore, the HOMO level ofDPQd can be determined to be −4.85−0.21=−5.06 [eV].

Note that when the HOMO levels of BPAPQ and DNTPD which were calculatedin the above-described manner are compared, it can be found that theHOMO level of DNTPD was lower than that of BPAPQ by as much as 0.48[eV]. This means that DNTPD can function as hole traps when added intoBPAPQ. Therefore, for the light-emitting element of the invention, it isquite advantageous to use the element structure shown in Embodiment 6 inwhich DNTPD is used as the second organic compound of the second layerand BPAPQ is used as the first organic compound thereof.

The present application is based on Japanese Priority applications No.2006-184653 filed on Jul. 4, 2006; No. 2006-327610 filed on Dec. 4,2006; and No. 2007-073089 filed on Mar. 20, 2007 with the JapanesePatent Office, the entire contents of which are hereby incorporated byreference.

1. A light-emitting device comprising: a first electrode; a secondelectrode; a light-emitting layer formed between the first electrode andthe second electrode; and a layer for controlling a movement of carriersformed between the light-emitting layer and the second electrode,wherein the layer for controlling the movement of carriers contains afirst organic compound and a second organic compound; wherein the firstorganic compound has an electron transporting property; wherein thesecond organic compound has an electron trapping property; wherein aweight percent of the first organic compound is higher than a weightpercent of the second organic compound in the layer for controlling themovement of carriers; and wherein the light-emitting layer emits lightwhen a voltage is applied such that a potential of the first electrodeis higher than a potential of the second electrode.
 2. Thelight-emitting device according to claim 1, wherein a lowest unoccupiedmolecular orbital level of the second organic compound is lower than alowest unoccupied molecular orbital level of the first organic compoundby 0.3 eV or more.
 3. The light-emitting device according to claim 1,wherein a thickness of the layer for controlling a movement of carriersis in a range of 5 nm to 20 nm.
 4. The light-emitting device accordingto claim 1, wherein a concentration of the second organic compound is ina range of 0.1 wt % to 5 wt % or in a range of 0.1 mol % to 5 mol %. 5.The light-emitting device according to claim 1, wherein thelight-emitting layer has an electron transporting property.
 6. Thelight-emitting device according to claim 1, wherein the light-emittinglayer contains a third organic compound and a fourth organic compound;wherein a weight percent of the third organic compound is higher than aweight percent of the fourth second organic compound; and wherein thethird organic compound has an electron transporting property.
 7. Thelight-emitting device according to claim 6, wherein the first organiccompound and the third organic compound are different organic compounds.8. The light-emitting device according to claims 1, wherein the firstorganic compound is a metal complex.
 9. The light-emitting deviceaccording to claim 1, wherein the second organic compound is a coumarinderivative.
 10. The light-emitting device according to claim 1, whereinthe light-emitting device is a lighting device.
 11. An electronic devicecomprising the light-emitting device according to claim
 1. 12. Theelectronic device according to claim 11, wherein the light-emittingdevice is provided at a display portion.
 13. A light-emitting devicecomprising: an anode; a cathode; a light-emitting layer formed betweenthe anode and the cathode; and a layer for controlling a movement ofcarriers formed between the light-emitting layer and the cathode,wherein the layer for controlling the movement of carriers contains afirst organic compound and a second organic compound; wherein the firstorganic compound has an electron transporting property; wherein thesecond organic compound has an electron trapping property; and wherein aweight percent of the first organic compound is higher than a weightpercent of the second organic compound in the layer for controlling themovement of carriers.
 14. The light-emitting device according to claim13, wherein a lowest unoccupied molecular orbital level of the secondorganic compound is lower than a lowest unoccupied molecular orbitallevel of the first organic compound by 0.3 eV or more.
 15. Thelight-emitting device according to claim 13, wherein a thickness of thelayer for controlling a movement of carriers is in a range of 5 nm to 20nm.
 16. The light-emitting device according to claim 13, wherein aconcentration of the second organic compound is in a range of 0.1 wt %to 5 wt % or in a range of 0.1 mol % to 5 mol %.
 17. The light-emittingdevice according to claim 13, wherein the light-emitting layer has anelectron transporting property.
 18. The light-emitting device accordingto claim 13, wherein the light-emitting layer contains a third organiccompound and a fourth organic compound; wherein a weight percent of thethird organic compound is higher than a weight percent of the fourthsecond organic compound; and wherein the third organic compound has anelectron transporting property.
 19. The light-emitting device accordingto claim 18, wherein the first organic compound and the third organiccompound are different organic compounds.
 20. The light-emitting deviceaccording to claims 13, wherein the first organic compound is a metalcomplex.
 21. The light-emitting device according to claim 13, whereinthe second organic compound is a coumarin derivative.
 22. Thelight-emitting device according to claim 13, wherein the layer forcontrolling the movement of carriers is in contact with thelight-emitting layer.
 23. The light-emitting device according to claim13, wherein the light-emitting device is a lighting device.
 24. Anelectronic device comprising the light-emitting device according toclaim
 13. 25. The electronic device according to claim 24, wherein thelight-emitting device is provided at a display portion.
 26. Alight-emitting device comprising: a first electrode; a second electrode;a light-emitting layer formed between the first electrode and the secondelectrode; and a layer for controlling a movement of carriers formedbetween the light-emitting layer and the first electrode, wherein thelayer for controlling the movement of carriers contains a first organiccompound and a second organic compound; wherein the first organiccompound has a hole transporting property; wherein the second organiccompound has a hole trapping property; wherein a weight percent of thefirst organic compound is higher than a weight percent of the secondorganic compound in the layer for controlling the movement of carriers;and wherein the light-emitting layer emits light when a voltage isapplied such that a potential of the first electrode is higher than apotential of the second electrode.
 27. The light-emitting deviceaccording to claim 26, wherein a highest unoccupied molecular orbitallevel of the second organic compound is higher than a highest unoccupiedmolecular orbital level of the first organic compound by 0.3 eV or more.28. The light-emitting device according to claim 26, wherein a thicknessof the layer for controlling a movement of carriers is in a range of 5nm to 20 nm.
 29. The light-emitting device according to claim 26,wherein a concentration of the second organic compound is in a range of0.1 wt % to 5 wt % or in a range of 0.1 mol % to 5 mol %.
 30. Thelight-emitting device according to claim 26, wherein the light-emittinglayer has a hole transporting property.
 31. The light-emitting deviceaccording to claim 26, wherein the light-emitting layer contains a thirdorganic compound and a fourth organic compound; wherein a weight percentof the third organic compound is higher than a weight percent of thefourth organic compound; and wherein the third organic compound has ahole transporting property.
 32. The light-emitting device according toclaim 31, wherein the first organic compound and the third organiccompound are different organic compounds.
 33. The light-emitting deviceaccording to claim 26, wherein the first organic compound is an aromaticamine compound.
 34. The light-emitting device according to claim 26,wherein the light-emitting device is a lighting device.
 35. Anelectronic device comprising the light-emitting device according toclaim
 26. 36. The electronic device according to claim 35, wherein thelight-emitting device is provided at a display portion.
 37. Alight-emitting device comprising: an anode; a cathode; a light-emittinglayer formed between the anode and the cathode; and a layer forcontrolling a movement of carriers formed between the light-emittinglayer and the anode, wherein the layer for controlling the movement ofcarriers contains a first organic compound and a second organiccompound; wherein the first organic compound has a hole transportingproperty; wherein the second organic compound has a hole trappingproperty; and wherein a weight percent of the first organic compound ishigher than a weight percent of the second organic compound in the layerfor controlling the movement of carriers.
 38. The light-emitting deviceaccording to claim 37, wherein a highest unoccupied molecular orbitallevel of the second organic compound is higher than a highest unoccupiedmolecular orbital level of the first organic compound by 0.3 eV or more.39. The light-emitting device according to claim 37, wherein a thicknessof the layer for controlling a movement of carriers is in a range of 5nm to 20 nm.
 40. The light-emitting device according to claim 37,wherein a concentration of the second organic compound is in a range of0.1 wt % to 5 wt % or in a range of 0.1 mol % to 5 mol %.
 41. Thelight-emitting device according to claim 37, wherein the light-emittinglayer has a hole transporting property.
 42. The light-emitting deviceaccording to claim 37, wherein the light-emitting layer contains a thirdorganic compound and a fourth organic compound; wherein a weight percentof the third organic compound is higher than a weight percent of thefourth organic compound; and wherein the third organic compound has ahole transporting property.
 43. The light-emitting device according toclaim 42, wherein the first organic compound and the third organiccompound are different organic compounds.
 44. The light-emitting deviceaccording to claim 37, wherein the first organic compound is an aromaticamine compound.
 45. The light-emitting device according to claim 37,wherein the layer for controlling the movement of carriers is in contactwith the light-emitting layer.
 46. The light-emitting device accordingto claim 37, wherein the light-emitting device is a lighting device. 47.An electronic device comprising the light-emitting device according toclaim
 37. 48. The electronic device according to claim 47, wherein thelight-emitting device is provided at a display portion.